Changes in firmware/mpconfigport.h are minimal:
* $VFS_FAT is dropped because it is irrelevant without $VFS
* $REVERSE_SPECIAL_METHODS is added, to push down on code size
* $THREAD is disabled unconditionally
unix/mpconfigport.h is reordered and modified to match
firmware/mpconfigport.h as much as possible, while selectively enabling
some needed features.
Notably, $USE_READLINE is enabled unconditionally
