[RFC] Explore and decide on a bootloader option

[MabezDev]

As things currently stand, we've been using the ESP-IDF bootloader, this has historically been for simplicity, but also the fact that we get compatibility with much of the IDF tooling. There is a strong argument to stick with this, but before we make a decision we should evaluate the other options.

I am of the opinion, that other than the esp-idf bootloader, if we choose a bootloader it should be written in Rust, to avoid complicated build steps, or in the case of esp-idf's bootloader, integration with tooling (pre built bootloaders within espflash/probe-rs).

embassy-boot

embassy-boot (docs) is a lightweight bootloader which supports

• DFU upgrades (on chips with USB)
• OTA (one slot only it seems)
• Image signing
• Support is provided mostly through the embedded-storage traits, but may also require some glue code for specific startup code on some chips
• There isn't support for esp-idf's secure boot OOTB, but I believe this could be added with our implementation.

mcuboot-rs

mcuboot-rs is a implementation of mcuboot, but in Rust. It is currently in PoC status.

• Will eventually support all the features of mcuboot
  • This includes secure boot and signed images
• Only supports LPC55S69 right now

ROM bootloader image format

One unresolved issue is how to take a bootloader elf produced from Rust code, and get it into a format where by the ROM code bootloader (in non-direct boot mode) will understand, see this. Post build steps aren't really a thing in cargo, so we'd need a way to solve this. Unless of course we don't, we could just make the bootloader flashing a one-time step that a user has to do from time to time.