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
core: moved `#include`s outside the `extern "C"` guards
drivers: add `extern "C"` to header files
Merge pull request #1848 from locicontrols/cc2538-nvic-prio-bits

cc2538: Set __NVIC_PRIO_BITS to the correct value of 3.
byteorder: silence warnings in clang
Merge pull request #1852 from authmillenon/byteorder-silence

byteorder: silence warnings in clang
Merge pull request #1805 from BytesGalore/extern_C_in_headers_core_unguard_includes

core: moved `#include`s outside the `extern "C"` guards
Merge pull request #1807 from BytesGalore/extern_C_in_headers_drivers

drivers: add `extern "C"` to header files
boards: core: cpu: drivers: sys: added missing header guards