Merge pull request #9290 from PeterKietzmann/pr_sram_puf

Makefile.dep

cpu/cc2538/Makefile.features

cpu/cortexm_common/ldscripts/cortexm_base.ld

cpu/cortexm_common/vectors_cortexm.c

cpu/samd21/Makefile.features

cpu/stm32f4/Makefile.features

sys/include/puf_sram.h

sys/puf_sram/Makefile

sys/puf_sram/puf_sram.c

sys/random/random.c

tests/puf_sram/Makefile

tests/puf_sram/README.md

tests/puf_sram/main.c

tests/puf_sram/tests/example_test.py

tests/puf_sram/tests/puf_sram_if.py