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 and MICROPY_PY_ASYNCIO.
Signed-off-by: Rick Sorensen <rick.sorensen@gmail.com>
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