Allow changing size of the filesystem on samd21 per board

As the CircuitPython grows, it may at some point no longer be possible
to squeeze it on some of the flash-less SAMD21 boards, so it would be
good to have the option of shrinking the filesystem size on per-board
basis. This change allows to define CIRCUITPY_INTERNAL_FLASH_FILESYSTEM_SIZE
inside the board's mpconfigborard.h file.

Author: deshipu

ports/atmel-samd/mpconfigport.h

@@ -92,11 +92,13 @@
 
 #ifdef SAMD21
 
+#ifndef CIRCUITPY_INTERNAL_FLASH_FILESYSTEM_SIZE
 #if INTERNAL_FLASH_FILESYSTEM
 #define CIRCUITPY_INTERNAL_FLASH_FILESYSTEM_SIZE (64 * 1024)
 #else
 #define CIRCUITPY_INTERNAL_FLASH_FILESYSTEM_SIZE (0)
 #endif
+#endif
 
 #ifndef CIRCUITPY_INTERNAL_NVM_SIZE
 #define CIRCUITPY_INTERNAL_NVM_SIZE (256)