* atmel-samd: Support patching after updating ASF4.
This makes it possible to automatically fix newer code.
* atmel-samd: Update ASF4 to include flash APIs for SAMD51.
This is the first automatic update that has caused a few deletions
where code was previously copied instead of moved.
This is a prerequisite for #260.
* atmel-samd: Remove ASF3. This will break builds.
* atmel-samd: Add ASF4 for the SAMD21 and SAMD51.
* Introduce the supervisor concept to facilitate porting.
The supervisor is the code which runs individual MicroPython VMs. By
splitting it out we make it more consistent and easier to find.
This also adds very basic SAMD21 and SAMD51 support using the
supervisor. Only the REPL currently works.
This begins the work for #178.
