recovery: Fix adb with linux 3.10

In kernel 3.10, f_adb has been removed and adbd can use functionfs
instead. Mount functionfs on boot for adbd. On older kernels, mount
will fail silently and adbd will revert to f_adb.

Change-Id: I5db57aaf35b35859ea88c7d0e0661d8c553e5811