cmake: Don't use the environment variable CFLAGS

Some distros set the environment variable CFLAGS, this will
accidentally affect Zephyr builds.

To fix this we clear the environment variable from within the Zephyr
build system for the duration of the CMake execution.

Until now we have been instructing the user to clear it, but it is
easier for the user if we clear it for him.

The same applies to CXXFLAGS.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>