make: del no-delete-null-pointer-checks for clang++

This PR enables the use of clang++ to compile C++ projects by adding
`CXXUWFLAGS += -fno-delete-null-pointer-checks`, if `${CC}` understands
`-fno-delete-null-pointer-checks` but `${CXX}` does not.
netdev.tests: Add fixes for environment variables

+ minor include fix
netdev.tests: Kill processes properly
cpu: Implement SPI driver for sam3x8e
boards: Implement SPI definitions for arduino-due
Merge pull request #1778 from authmillenon/netdev-test-env-fixes

netdev.tests: Add fixes for environment variables
Merge pull request #1584 from PeterKietzmann/add_sam3x_spi

cpu/board: updated  spi driver for arduino-due
native/freebsd: fix tapsetup permission
native/freebsd: fix usecond typdef
Merge pull request #1729 from Kijewski/clang++

make: del no-delete-null-pointer-checks for clang++
nativenet: Apply coding conventions
nativenet: Supply net_dev_t support for nativenet
netdev.tests: Remove unneccessary #ifndef path
Merge pull request #1732 from authmillenon/netdev-nativenet

nativenet: Port nativenet for netdev
Merge pull request #1780 from LudwigOrtmann/native-freebsd-fixup

sys/posix: FreeBSD fixup
drivers: Make general IEEE 802.15.4 driver part of netdev
Merge branch 'netdev-test-dummy' into HEAD
Merge branch 'netdev-ieee802154' into HEAD
sys: net: Initial import of a general interface to a network protocol
sys: net: Initial import of a basic MAC protocol layer