ADB sideload command not work

In kernel(3.10) USB ADB gadget driver is removed.
Using Functionfs for USB adb gadget.

Android recovery uses a stripped down version
of adb command for sideload only. It's missing
the ffs function support, so add the ffs
support to allow sideload command to work

b/12608946

Change-Id: I4ad024723dfc5bdb544548391f99637c390b171e
Signed-off-by: Pierre Couillaud <pierre@broadcom.com>
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