Implement application description for ESP-IDF image format

[jessebraham]

Details can be found in the ESP-IDF documentation:
https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-reference/system/app_image_format.html#application-description

The omission of this struct can cause issues when using newer versions of the bootloader.

[SergioGasquez]

Here is my progress with this issue: https://github.com/SergioGasquez/espflash/tree/feat/app-desc. Not sure what's wrong, but the app bootloops because the checksum verification fails or because there is an invalid segment length.

Note that I've only updated the C6 bootloader