build: Update to Makefile.cmake to help devicetree development

Adds new options to Makefile:

Target:-
generate: just do the generation stage of CMake - this is sufficient to compile
the device trees

Input Options:
CPP_ONLY: Compile target but stop after C pre-processor stage. Allows examination of
structures created by the devicetree macros

BUILD_TARGET: Create only a single specified target for the product

Signed-off-by: Mike Leach <mike.leach@linaro.org>