samd/mcu/samd21/mpconfigmcu.h:Allow user to enable additional options.

[ricksorensen]

Currently for samd21 some features are disable because of limited memory. With the ability to trade firmware and filesystem space, a user may wish to selectively enable some of these features. This change allows them to be enabled in board mpconfigboard.h or on the build command line for example. The selectively enable functions are:

• MICROPY_PY_FRAMEBUF
• MICROPY_PY_SELECT
• MICROPY_PY_ONEWIRE
• MICROPY_PY_ASYNCIO

[robert-hh]

That's fine. As part of the make process also the setting for MICROPY_HW_CODESIZE has to be adapted accordingly.
Note: The commit message need a space character after the colon. Just change the commit locally with git commit --amend and then push it with git push <your_forks_name> samd_func -f.

[github-actions]

Code size report:

   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:    +0 +0.000% standard
      stm32:    +0 +0.000% PYBV10
     mimxrt:    +0 +0.000% TEENSY40
        rp2:    +0 +0.000% RPI_PICO_W
       samd:    +0 +0.000% ADAFRUIT_ITSYBITSY_M4_EXPRESS