esp32: Add support for noinit variables in SPIRAM

Add Kconfig option SPIRAM_ALLOW_NOINIT_EXTERNAL_MEMORY
When enabled, a new linker script rule (from esp32.extram.noinit.ld)
places any variables in the .ext_ram.noinit section in SPIRAM.

This section is exempted from the startup SPIRAM memory test and is
not zero-initialized or added to the malloc pool, making it usable
for noinit variables that persist across reset.

The EXT_RAM_NOINIT_ATTR macro places variables in this section.

components/esp_common/include/esp_attr.h

@@ -72,6 +72,14 @@ extern "C" {
 #define EXT_RAM_ATTR
 #endif
 
+#if CONFIG_SPIRAM_ALLOW_NOINIT_EXTERNAL_MEMORY
+// Forces data into external memory noinit section to avoid initialization after restart.
+#define EXT_RAM_NOINIT_ATTR _SECTION_ATTR_IMPL(".ext_ram.noinit", __COUNTER__)
+#else
+// Place in internal noinit section
+#define EXT_RAM_NOINIT_ATTR __NOINIT_ATTR
+#endif
+
 // Forces data into RTC slow memory. See "docs/deep-sleep-stub.rst"
 // Any variable marked with this attribute will keep its value
 // during a deep sleep / wake cycle.
@@ -155,4 +163,3 @@ FORCE_INLINE_ATTR TYPE& operator<<=(TYPE& a, int b) { a <<= b; return a; }
 }
 #endif
 #endif /* __ESP_ATTR_H__ */
-