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
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
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
Merge pull request #1779 from thomaseichinger/stm32f1-flashing

boards/iot-lab_M3: change to openocd's new ftdi interface
stm32f1: implement RTT driver
Merge pull request #1613 from thomaseichinger/stm32f1_rtt

stm32f1:iot-lab_M3: implement RTT driver
sixlowpan: fix ipv6_addr_init_prefix
Merge pull request #1781 from authmillenon/sixlowpan-fix-prefix-init

sixlowpan: fix ipv6_addr_init_prefix
Merge pull request #1783 from dangnhat/extern_C_in_headers_core

c++: core: add extern C in header files