cpu: Add a way to use microcode .h files back to the build

cpu: Add a way to use microcode .h files back to the build

The build was changed to remove usage of microcode .h files when
all of the .h files were converted to binary.  This is still
needed for some builds when microcode binaries aren't in the
blobs tree.

Change-Id: Ia323c90efe8aa0b8799fc5cce6197509e466a105
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: http://review.coreboot.org/12333
Tested-by: Raptor Engineering Automated Test Stand <noreply@raptorengineeringinc.com>
Reviewed-by: Timothy Pearson <tpearson@raptorengineeringinc.com>
Tested-by: build bot (Jenkins)
Reviewed-by: Werner Zeh <werner.zeh@siemens.com>