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
Initially supports only Mulles with serial number > 220 (due to missing
MK60DN256ZVLL10 support in k60).
See also: https://github.com/RIOT-OS/RIOT/wiki/Board%3A-Mulle
Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
