There's a number of reasons to use an alternate libril:
* Older RIL blobs with incompatible ABIs
* Vendors that don't quite follow the documented API
* Devices derived from CAF code (which has some differences to
Google's versions)
* Probably others...

If a BoardConfig includes a TARGET_PROVIDES_LIBRIL, this
will make it build libril using the code that's in <device-dir>/libril
instead of hardware/ril/libril.

Alternatively, that variable can contain the full path to the
alternative libril

Change-Id: I622e16964ed026805fb82377dc8240b04f53f467