libril: allow board to provide libril

Change-Id: If0033865ab179854c27c56eaaf078fcc30ea102a
libril: Provide support for legacy ril

With change Icc620fd191f8542f61bd38e365813753bfedefdf, RIL_VERSION_MIN was
increased from 2 to 6, but this causes problems for a number of devices
using ril blobs older than that.

  E/RILC    (  140): RIL_register: version 3 is to old, min version is 6

Consequently causing the ril system unable to connect to the rild socket.

Change-Id: I82c1cc4dab8fcaa4ccaa099132133b0f3b1d8029