core: align stack on a 32bit boundary

Fixes #1267.
make: don't overwrite LINKFLAGS

Fix the boards' Makefiles were a potentially set LINKFLAGS variable were
overwritten.
native: add periph substructure
native: relocate periph/cpuid implementation
native: add drivers substructure
native: relocate drivers/rtc implementation
Merge pull request #1833 from OlegHahm/dont_overwrite_linkflags

make: don't overwrite LINKFLAGS
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
ip: set solicited node address correctly
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
Merge pull request #1853 from authmillenon/solicited-node-address-fix

ip: set solicited node address correctly
lint: fix unassignedVariable warnings in stm32f1