Allow a device to include its own RIL subclass

For cases where a single device needs a subclass, there's no need to
pollute the common core with it: just stick it in the device's own
repository, and add something like
"BOARD_RIL_CLASS := ../../../device/manuf/model/ril/" to the BoardConfig

Change-Id: I9d01ceb6e5587382eb216341e2e373ec0c52848f