* atmel-samd: Add support for internal filesystems.
This allows us to re-enable `os`. `random` is also enabled because
it solely depends on `os`.
Fixes#266. Its also a pre-requisite for #260.
* atmel-samd: Update SAMD51 linker script comments and MICROPY_MAX_STACK_USAGE enabling.
* 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.
