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
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