core: align stack on a 32bit boundary

Fixes #1267.
native: add periph substructure
native: relocate periph/cpuid implementation
native: add drivers substructure
native: relocate drivers/rtc implementation
cc2538: Set __NVIC_PRIO_BITS to the correct value of 3.
Merge pull request #1268 from Kijewski/issue-1267

core: align stack on a 32bit boundary
Merge pull request #1838 from LudwigOrtmann/native_subsstructure

native: add substructure
Merge pull request #1848 from locicontrols/cc2538-nvic-prio-bits

cc2538: Set __NVIC_PRIO_BITS to the correct value of 3.