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.
nativenet: Apply coding conventions
nativenet: Supply net_dev_t support for nativenet
netdev.tests: Remove unneccessary #ifndef path
native/freebsd: fix tapsetup permission
native/freebsd: fix usecond typdef
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.
stm32f1: implement RTT driver
[SQASH ME] removed spi transfers during ISR