This refactoring of the chip specific bootloader creates the internal boot instance and aligned
buffer in the prepare stage, so that they are automatically dropped after. This unlocks a use
case where peripherals owning the flash need to be Drop'ed before load() happens.

By passing WDT config around we can control it more easily and promote sharing it between files.

Compiler will infer a different lifetime for BootFlash than for the
borrowed flash, which makes it require more type annotations than if it
was just owning the type. Since it doesn't really matter if it owns or
borrows in practical use, change it to own so that it simplifies usage.

* 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