Merge pull request #9505 from PeterKietzmann/pr_puf_stm32

cpu/stm32_common: add PUF SRAM feature to makefile

cpu/stm32_common/Makefile.features

 FEATURES_PROVIDED += periph_cpuid
+FEATURES_PROVIDED += puf_sram
 
 ifneq (,$(filter $(BOARDS_WITHOUT_HWRNG),$(BOARD)))
   FEATURES_PROVIDED := $(filter-out periph_hwrng,$(FEATURES_PROVIDED))

cpu/stm32f4/Makefile.features

 FEATURES_PROVIDED += periph_hwrng
-FEATURES_PROVIDED += puf_sram
 
 # the granularity of provided feature definition for STMs is currently by CPU
 # sub-family (e.g., stm32f[1234]).  Unfortunately, only some of e.g., the