app_update: Add API for getting sha256_of_partition

Added bootloader_common_get_sha256_of_partition() and esp_partition_get_sha256() - get or calculate SHA-256
digest for app and data partitions.
Added bootloader_sha256_hex_to_str() - helps to print SHA-256 digest
Added esp_partition_check_identity() - compares two partitions by SHA-256 digest

Refactoring a function esp_image_load() in bootloader space to esp_image_verify() and
bootloader_load_image(). Old name function esp_image_load is deprecated
and will remove in V4.0 version.

spi_flash/sim: Fix error test_host. Add stub for bootloader_common_get_sha256_of_partition in sim/stubs

components/spi_flash/sim/stubs/bootloader_support/src/bootloader_common.c

@@ -0,0 +1,19 @@
+// Copyright 2018 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.
+#include "esp_err.h"
+
+esp_err_t bootloader_common_get_sha256_of_partition (uint32_t address, uint32_t size, int type, uint8_t *out_sha_256)
+{
+    return ESP_OK;
+}