Docs: Document flash encryption disable steps, clean up flash encryption & secure boot docs
Generate clean up of flash encryption & secure boot docs
Including steps for disabling flash encryption (for people who accidentally enable it).
See merge request !500
Also add steps to disable flash encryption, as some people seem to
accidentally enable it.
Explicitly mark the flash encryption and secure boot as "READ DOCS
FIRST" in menuconfig.
Seems doing certain kinds of short reads while flash encryption is
enabled will return stale data. This fixes it, but is probably a
little heavy-handed performance wise.
* App access functions are all flash encryption-aware
* Documentation for flash encryption
* Partition read/write is flash aware
* New encrypted write function
