overo.conf: support meta-ti ti-dsplink builds

Suggestion from http://www.sleepyrobot.com/?p=210 via
http://gumstix.8.n6.nabble.com/gles-and-dsp-support-in-the-official-yocto-layer-tc4965764.html

Signed-off-by: Peter A. Bigot <pab@pabigot.com>
Merge pull request #3 from pabigot/for-dylan/dspbuild

overo.conf: support meta-ti ti-dsplink builds