cmake: Check if match between CMAKE_BUILD_TYPE and OPTIMIZATION_FLAG

In CMake projects, 'CMAKE_BUILD_TYPE' usually determines the
optimization flag, but in Zephyr it is determined through Kconfig.

To avoid confusing users we now give a warning if there is a mismatch
between the CMAKE_BUILD_TYPE and the optimization flag.

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