Enable secure boot only after encrypting flash

This prevents a device from being bricked in case when both secure boot & flash encryption are enabled and encryption gets interrupted during first boot. After interruption, all partitions on the device need to be reflashed (including the bootloader).

List of changes:
* Secure boot key generation and bootloader digest generation logic, implemented inside function esp_secure_boot_permanently_enable(), has been pulled out into new API esp_secure_boot_generate_digest(). The enabling of R/W protection of secure boot key on EFUSE still happens inside esp_secure_boot_permanently_enable()
* Now esp_secure_boot_permanently_enable() is called only after flash encryption process completes
* esp_secure_boot_generate_digest() is called before flash encryption process starts

components/bootloader_support/src/bootloader_utility.c

@@ -478,24 +478,71 @@ void bootloader_utility_load_boot_image(const bootloader_state_t *bs, int start_
 // Copy loaded segments to RAM, set up caches for mapped segments, and start application.
 static void load_image(const esp_image_metadata_t* image_data)
 {
+    /**
+     * Rough steps for a first boot, when encryption and secure boot are both disabled:
+     *   1) Generate secure boot key and write to EFUSE.
+     *   2) Write plaintext digest based on plaintext bootloader
+     *   3) Generate flash encryption key and write to EFUSE.
+     *   4) Encrypt flash in-place including bootloader, then digest,
+     *      then app partitions and other encrypted partitions
+     *   5) Burn EFUSE to enable flash encryption (FLASH_CRYPT_CNT)
+     *   6) Burn EFUSE to enable secure boot (ABS_DONE_0)
+     *
+     * If power failure happens during Step 1, probably the next boot will continue from Step 2.
+     * There is some small chance that EFUSEs will be part-way through being written so will be
+     * somehow corrupted here. Thankfully this window of time is very small, but if that's the
+     * case, one has to use the espefuse tool to manually set the remaining bits and enable R/W
+     * protection. Once the relevant EFUSE bits are set and R/W protected, Step 1 will be skipped
+     * successfully on further reboots.
+     *
+     * If power failure happens during Step 2, Step 1 will be skipped and Step 2 repeated:
+     * the digest will get re-written on the next boot.
+     *
+     * If power failure happens during Step 3, it's possible that EFUSE was partially written
+     * with the generated flash encryption key, though the time window for that would again
+     * be very small. On reboot, Step 1 will be skipped and Step 2 repeated, though, Step 3
+     * may fail due to the above mentioned reason, in which case, one has to use the espefuse
+     * tool to manually set the remaining bits and enable R/W protection. Once the relevant EFUSE
+     * bits are set and R/W protected, Step 3 will be skipped successfully on further reboots.
+     *
+     * If power failure happens after start of 4 and before end of 5, the next boot will fail
+     * (bootloader header is encrypted and flash encryption isn't enabled yet, so it looks like
+     * noise to the ROM bootloader). The check in the ROM is pretty basic so if the first byte of
+     * ciphertext happens to be the magic byte E9 then it may try to boot, but it will definitely
+     * crash (no chance that the remaining ciphertext will look like a valid bootloader image).
+     * Only solution is to reflash with all plaintext and the whole process starts again: skips
+     * Step 1, repeats Step 2, skips Step 3, etc.
+     *
+     * If power failure happens after 5 but before 6, the device will reboot with flash
+     * encryption on and will regenerate an encrypted digest in Step 2. This should still
+     * be valid as the input data for the digest is read via flash cache (so will be decrypted)
+     * and the code in secure_boot_generate() tells bootloader_flash_write() to encrypt the data
+     * on write if flash encryption is enabled. Steps 3 - 5 are skipped (encryption already on),
+     * then Step 6 enables secure boot.
+     */
+
 #if defined(CONFIG_SECURE_BOOT_ENABLED) || defined(CONFIG_FLASH_ENCRYPTION_ENABLED)
     esp_err_t err;
 #endif
+
 #ifdef CONFIG_SECURE_BOOT_ENABLED
-    /* Generate secure digest from this bootloader to protect future
-       modifications */
-    ESP_LOGI(TAG, "Checking secure boot...");
-    err = esp_secure_boot_permanently_enable();
+    /* Steps 1 & 2 (see above for full description):
+     *   1) Generate secure boot EFUSE key
+     *   2) Compute digest of plaintext bootloader
+     */
+    err = esp_secure_boot_generate_digest();
     if (err != ESP_OK) {
-        ESP_LOGE(TAG, "Bootloader digest generation failed (%d). SECURE BOOT IS NOT ENABLED.", err);
-        /* Allow booting to continue, as the failure is probably
-           due to user-configured EFUSEs for testing...
-        */
+        ESP_LOGE(TAG, "Bootloader digest generation for secure boot failed (%d).", err);
+        return;
     }
 #endif
 
 #ifdef CONFIG_FLASH_ENCRYPTION_ENABLED
-    /* encrypt flash */
+    /* Steps 3, 4 & 5 (see above for full description):
+     *   3) Generate flash encryption EFUSE key
+     *   4) Encrypt flash contents
+     *   5) Burn EFUSE to enable flash encryption
+     */
     ESP_LOGI(TAG, "Checking flash encryption...");
     bool flash_encryption_enabled = esp_flash_encryption_enabled();
     err = esp_flash_encrypt_check_and_update();
@@ -503,7 +550,23 @@ static void load_image(const esp_image_metadata_t* image_data)
         ESP_LOGE(TAG, "Flash encryption check failed (%d).", err);
         return;
     }
+#endif
 
+#ifdef CONFIG_SECURE_BOOT_ENABLED
+    /* Step 6 (see above for full description):
+     *   6) Burn EFUSE to enable secure boot
+     */
+    ESP_LOGI(TAG, "Checking secure boot...");
+    err = esp_secure_boot_permanently_enable();
+    if (err != ESP_OK) {
+        ESP_LOGE(TAG, "FAILED TO ENABLE SECURE BOOT (%d).", err);
+        /* Allow booting to continue, as the failure is probably
+           due to user-configured EFUSEs for testing...
+        */
+    }
+#endif
+
+#ifdef CONFIG_FLASH_ENCRYPTION_ENABLED
     if (!flash_encryption_enabled && esp_flash_encryption_enabled()) {
         /* Flash encryption was just enabled for the first time,
            so issue a system reset to ensure flash encryption