build: Allow devices to specify a RIL variant

Change-Id: Ia5d35d916be358fb7be7ead6bd05cc3628b35a37
core: Get the RIL variant path into the global includes

 * Need to have this in the global path, so shuffle things around a little.

Change-Id: I2dd2b378d17ff3fa0e379793df31d68964d87278