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.
boards + cpu: atmega2560: fix non-existent GPIO
drivers/periph/gpio: fix #ifdef instead of #if
Merge pull request #1831 from LudwigOrtmann/periph_gpio_ifdef

drivers/periph/gpio: fix #ifdef instead of #if
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
native: implement dummy GPIO interface
tests: driver_pir: fix unused argument warning
tests: driver_pir: configure for native
native: add rudimentary sysfs gpio driver
FIXUP: allow external definition of sysfs GPIOs
native: add "open" syscall wrapper
FIXUP:  guard native system calls
tests: enable native in periph_gpio
FIXUP: remove silly modes
FIXUP: test/periph_gpio: sysfs example in Makefile
FIXUP: add sysfs writing, style & fixes
SQUASHME: rename dummy -> virtual