Allows OTA updates to be secured via signature checks, without requiring the overhead or complexity
of a full secure boot implementation.
Uses same signing mechanisms (build system and/or espsecure.py as Secure Boot).
Requires:
* [ ] More testing
* [ ] Documentation
Makes app image booting more reliable (256-bit rather than 8-bit verification.)
Some measurements, time to boot a 655KB app.bin file and run to app_main() execution.
(All for rev 1 silicon, ie no 340ms spurious WDT delay.)
80MHz QIO mode:
before = 300ms
after = 140ms
40MHz DIO mode:
before = 712ms
after = 577ms
40MHz DIO mode, secure boot enabled
before = 1380ms
after = 934ms
(Secure boot involves two ECC signature verifications (partition table, app) that take approx 300ms each with 80MHz CPU.)
* App access functions are all flash encryption-aware
* Documentation for flash encryption
* Partition read/write is flash aware
* New encrypted write function
