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

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>

ports/samd/mcu/samd21/mpconfigmcu.h

@@ -63,12 +63,20 @@ unsigned long trng_random_u32(int delay);
 #define MICROPY_PY_BUILTINS_ROUND_INT   (SAMD21_EXTRA_FEATURES)
 #define MICROPY_CAN_OVERRIDE_BUILTINS   (SAMD21_EXTRA_FEATURES)
 #define MICROPY_PY_SYS_STDIO_BUFFER     (SAMD21_EXTRA_FEATURES)
+#ifndef MICROPY_PY_FRAMEBUF
 #define MICROPY_PY_FRAMEBUF             (SAMD21_EXTRA_FEATURES)
+#endif
+#ifndef MICROPY_PY_ASYNCIO
 #define MICROPY_PY_ASYNCIO              (SAMD21_EXTRA_FEATURES)
+#endif
+#ifndef MICROPY_PY_SELECT
 #define MICROPY_PY_SELECT               (SAMD21_EXTRA_FEATURES)
+#endif
 #define MICROPY_PY_ERRNO                (SAMD21_EXTRA_FEATURES)
 #define MICROPY_PY_DEFLATE              (SAMD21_EXTRA_FEATURES)
+#ifndef MICROPY_PY_ONEWIRE
 #define MICROPY_PY_ONEWIRE              (SAMD21_EXTRA_FEATURES)
+#endif
 
 #ifndef MICROPY_PY_MACHINE_PIN_BOARD_CPU
 #define MICROPY_PY_MACHINE_PIN_BOARD_CPU (1)