Support flash encryption

[jessebraham]

Flash encryption is supported by all chips from the ESP32 and newer. We can encrypt data on the host and flash it to the target, assuming we first burn the encryption key into the target's eFuse.

For more details:
https://docs.espressif.com/projects/esp-idf/en/latest/esp32/security/flash-encryption.html

[jessebraham]

After some further discussion, it seems there's not really anything actionable here. I have written a short guide for using flash encryption in this gist:
https://gist.github.com/jessebraham/0c7d86f9866614dc602e5d8e0e97c37a

If there is any specific functionality we need in the future we can open issues for that specifically.

[ost-ing]

@jessebraham in your gist, you need to manually build a secure flash bootloader with esp-idf idf.py, and then manually pass that bootloader binary to espflash.

That seems fairly incomplete to me. Shouldn't espflash be able to prepare the bootloader for encryption?