cmake: Add option for exporting build metadata to Make

Add an opt-in feature that will generate a Makefile with build
variables like CC, and KBUILD_CFLAGS for consumption by third-party
Make-based build systems.

This emulates the 'outputexports' target that KBuild supported and is
supported for the same reasons that KBuild supported it. Easier
integration with third-party build systems.

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