Add the config directory to the rtcd generation script. libvpx is
configured in the jni directory but ndk-build is intended to be run from
the next directory up. Currently it needs to be run from the jni
directory but this is being looked in to.

Add a trailing slash to allow the variable to be empty.

Reduce offset generation to the files which are actually used.

Change-Id: Ia84fac37e8998ba647423d0ee45fc66a891ce10c