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.
Add a command `make erase_ota` - erases otadata if it there is in the
partition table otherwise error 'Partition table does not have ota_data partition'.
Closes https://github.com/espressif/esp-idf/issues/1821
