Merge pull request #2848 from haukepetersen/fix_samr21_optimization

cpu/samd21: remove -O0 override

cpu/samd21/Makefile.include

@@ -6,7 +6,4 @@ export CFLAGS += -DDONT_USE_CMSIS_INIT
 # use the hwtimer compatibility layer
 USEMODULE += hwtimer_compat
 
-# TODO: remove once the CPU implementation is stable with -Os
-CFLAGS_OPT = -O0
-
 include $(RIOTCPU)/Makefile.include.cortexm_common