Flash encryption: Support enabling flash encryption in bootloader, app support

* App access functions are all flash encryption-aware
* Documentation for flash encryption
* Partition read/write is flash aware
* New encrypted write function

components/bootloader_support/include_priv/bootloader_random.h

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+// Copyright 2010-2016 Espressif Systems (Shanghai) PTE LTD
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#pragma once
+
+#include <stddef.h>
+
+/**
+ * @brief Fill buffer with 'length' random bytes
+ *
+ * @param buffer Pointer to buffer
+ * @param length This many bytes of random data will be copied to buffer
+ */
+void bootloader_fill_random(void *buffer, size_t length);