add EUI-64 support for nativenet
fix includes and shell commands for at86rf231
remove hint at non-existent example
use pkg-config to add valgrind include path

makes knowledge of VALGRIND_VALGRIND_H vs. VALGRIND_H optional
update Makefiles to point to all-valgrind target

Header path knowledge is not necessary anymore
Merge pull request #626 from LudwigOrtmann/issue_621

add EUI-64 support for nativenet
correct cc110x function names
Merge pull request #682 from LudwigOrtmann/gpioint-example

remove hint at non-existent example
Merge pull request #681 from OlegHahm/at86rf231_fixes

at86rf231: fix includes and shell commands
Merge pull request #684 from OlegHahm/avsextrem_spi_function_renaming

correct cc110x function names
Merge pull request #664 from Kijewski/conditional-fno-delete-null-pointer-checks

clang does not know fno-delete-null-pointer-checks
Merge pull request #666 from LudwigOrtmann/native_valgrind_config

Native valgrind config
Add USEPKG variable for Makefiles

Currently pkg/USING says one should use
`EXTERNAL+=$(RIOTBASE)/pkg/<pkg_name>` to enable PKG modules.
Using this line the PKG will be compiled but not linked.

This change adds a USEPKG variable to be used like
`USEPKG += <pkg_name>`, which looks less clumsy and gets the PKG linked
in the binary.
Use include path convention for PKGs

Packages have to define a Makefile.include target in pkg/%/Makefile.
pkg/%/Makefile.include will be loaded after all other dependencies.
Apply @authmillenon's patch