Fixes a problem with make buildtest if TARGET=mulle and no USB serial device connected:
$ make buildtest
Warning: no PORT set!
Building for Warning: .. failed
.../riot/Makefile.include:110: *** The specified board Warning: does not exist.. Stop.
.../riot/Makefile.include:110: *** The specified board Warning: does not exist.. Stop.
Building for no (no linking) .. failed
.../riot/Makefile.include:110: *** The specified board no does not exist.. Stop.
.../riot/Makefile.include:110: *** The specified board no does not exist.. Stop.
Building for PORT .. failed
.../riot/Makefile.include:110: *** The specified board PORT does not exist.. Stop.
.../riot/Makefile.include:110: *** The specified board PORT does not exist.. Stop.
Building for set! .. failed
.../riot/Makefile.include:110: *** The specified board set! does not exist.. Stop.
.../riot/Makefile.include:110: *** The specified board set! does not exist.. Stop.
Building for airfy-beacon (no linking) .. ^C