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.
core: Provide functions for different byte orders

Rationale: see #1586.
tests: Add unittests for byteorder.h
tests: fix bad copy+paste in tests-core.h
netdev: Provide maximum value of netdev_opt_t for further definitions
Merge pull request #1774 from authmillenon/netdev-opt-max

netdev: Provide maximum value of netdev_opt_t for further definitions
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
boards/iot-lab_M3: change to openocd's new ftdi interface

Openocd communication was done over a deprecated interface definition,
namely ft2232. Using the new ftdi interface speeds up flashing and
doesn't throw warnings anymore.
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++
c++: core: add extern C in header files
nativenet: Apply coding conventions
nativenet: Supply net_dev_t support for nativenet
netdev.tests: Remove unneccessary #ifndef path