FM Radio: add silabs si4709

adds JNI interface to silabs si4709 FM Radio chip used in the
international Samsung Galaxys S.
The kind of radio chip is controlled by BOARD_FM_DEVICE because
we have a bcm4329 wifi chip that would conflict with the other
definitions in the make file.

Change-Id: Ibbcd0b89d4c080f17ce27d4b5f47449718ff48c9