cmake: move testing of compile flag in environment to top CMakeLists.txt

This moves the test of compile flags set in environment from generic
toolchain to top-level ${ZEPHYR_BASE}/CMakeLists.txt.

In order to cleanup and make Zephyr CMake modules more generic then
the testing of specific compile flags and warn user is not really a
toolchain module responsible but a implementation detail of the project
itself as to how compile flags should be applied in a project.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>