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

[ivmarkov]

@SergioGasquez I wonder why are you guys implementing this structure as part of espflash itself?
My understanding is, it needs to be a part of the factory/OTA image itself?

In esp-idf-sys, we have a simple app_desc! macro that folks can use to embed their own esp_app_desc_t structure in the ELF executable, which grabs current date and time, project = crate name (from Cargo.toml), and project version = crate version from Cargo.toml.

Shouldn't we use a similar approach for baremetal?
I mean, if you upload via OTA a new app image, you want this new app image to have a new version of this structure anyway, so my understanding is this is not related to espflash, which does not play a role during OTA anyway.

[ivmarkov]

Here's the macro in esp-idf-sys: https://github.com/esp-rs/esp-idf-sys/blob/master/src/app_desc.rs#L2