935: Remove generic const expressions from embassy-boot r=lulf a=lulf
* Remove the need for generic const expressions and use buffers provided in the flash config.
* Extend embedded-storage traits to simplify generics.
* Document all public APIs
* Add toplevel README
* Expose AlignedBuffer type for convenience.
* Update examples
Co-authored-by: Ulf Lilleengen <lulf@redhat.com>