Convert to Android.bp

See build/soong/README.md for more information about Soong.

Removes BOARD_GPU_DRIVERS, which wasn't affecting anything, since none
of the HAVE_* macros are defined. Even if they were, we'd prefer to
compile all of them so that a single library can support multiple
boards.

Test: mmma external/libdrm
Change-Id: Ie01736bce6cf41e3da5040fe5341ade0634b5111