platform/mulle: Rename MULLE_BOARD_SERIAL_NUMBER -> MULLE_SERIAL

MULLE_BOARD_SERIAL_NUMBER was getting tedious to type on the command line when moving between different boards frequently.
Also MULLE_SERIAL is better wrt to consistency with PROGRAMMER_SERIAL.

Added a backwards compatibility clause in `Makefile.mulle` to cope with older environment settings still left in users' `.bashrc` files.