esp32: Add firmware version to app

Added a new structure esp_app_desc_t. It has info about firmware:
version, secure_version, project_name, time/date build and IDF version.
Added the ability to add a custom structure with a description of the firmware.

The esp_app_desc_t is located in fixed place in start of ROM secotor. It is located after structures esp_image_header_t and esp_image_segment_header_t.

app_version is filed from PROJECT_VER variable (if set in custom make file) or PROJECT_PATH/version.txt or git repo (git describe).

Add API to get app_desc from partition.

components/app_update/Kconfig.projbuild

@@ -0,0 +1,12 @@
+menu "Application manager"
+
+config APP_COMPILE_TIME_DATE
+    bool "Use time/date stamp for app"
+    default y
+	help
+        If set, then the app will be built with the current time/date stamp. It is stored in the app description structure.
+        If not set, time/date stamp will be excluded from app image.
+        This can be useful for getting the same binary image files made from the same source, 
+        but at different times.
+
+endmenu # "Application manager"