ESP32-C3: Add support for booting from MCUboot bootloader

[gustavonihei]

Summary

This PR intends to enable an esp-hal-based Rust application to be booted from the MCUboot bootloader on ESP32-C3.

MCUboot is a secure bootloader solution feature-wise equivalent to the IDF 2nd stage bootloader (current default bootloader).
You may find more information on the documentation pages for MCUboot and the Espressif port:
https://docs.mcuboot.com/
https://docs.mcuboot.com/readme-espressif.html

Testing

Hello World example successfully executed from MCUboot bootloader on ESP32-C3-DevKitM-1 board according to instructions from the updated README.md file.

[jessebraham]

Just a couple small comments, otherwise as far as the code is concerned this looks fine to me. @bjoernQ would you mind taking a quick look as well please whenever you get a chance (if you haven't already)?

If you wouldn't mind it would also be nice for there to be a small section in the README briefly describing how to build and use this format. This isn't required, so if you're not feeling like doing it that's fine too, but would be appreciated!

[jessebraham]

Tested with the fixes to the linker script and everything is working as expected on my end, so once the review points have been addressed this should be good to merge!

[gustavonihei]

If you wouldn't mind it would also be nice for there to be a small section in the README briefly describing how to build and use this format.

Done! Created a new section on the README.md file covering not only MCUboot, but also briefly describing about the other existing booting alternatives.

https://github.com/esp-rs/esp-hal/blob/1ced513d969b92419901c5e4b6bb95cec5bf4a7d/esp32c3-hal/README.md#supported-boot-methods

[jessebraham]

README looks great, thanks! I think we're good to go here, so thank you again for your patience and your continued work on this feature, I look forward to expanding on it in the future!