boards should use rm -rf to clean up #419
Description
this is currently happening:
rmdir -p bin/
rmdir: failed to remove 'bin/': Directory not empty
make[2]: [clean] Error 1 (ignored)
$ git grep rmdir
boards/avsextrem/Makefile: then rmdir $(BINDIR) ; \
boards/chronos/Makefile: -rmdir $(BINDIR)
boards/msb-430/Makefile: then rmdir $(BINDIR) ; \
boards/msb-430h/Makefile: then rmdir $(BINDIR) ; \
boards/msba2/Makefile: then rmdir $(BINDIR) ; \
boards/native/Makefile: -rmdir -p $(BINDIR)
boards/redbee-econotag/Makefile: then rmdir $(BINDIR) ; \
boards/telosb/Makefile: then rmdir $(BINDIR) ; \
boards/wsn430-v1_3b/Makefile: then rmdir $(BINDIR) ; \
boards/wsn430-v1_4/Makefile: then rmdir $(BINDIR) ; \