Add a new build target called "cmupdaterincremental".

This can be used to generate an incremental update after make bacon (or
any other target that compiles a full build) was run.
Example command:
INCREMENTAL_SOURCE_BUILD_ID=f1785074ab INCREMENTAL_SOURCE_TARGETFILES_ZIP=out/target/product/mb526/obj/PACKAGING/target_files_intermediates/cm_mb526-target_files-f1785074ab.zip WITHOUT_CHECK_API=true ONE_SHOT_MAKEFILE=cmupdaterincremental make cmupdaterincremental

Change-Id: Ia193e7bf437a254e9b32e87b7b370c7e265a4827