package/gst1-libav: disable on avr32

gst1-libav produces particularly large binaries, and the relocations
needed for it do not fit in the possible relocation mechanisms
available on avr32. Let's just disable it.

Fixes:
    http://autobuild.buildroot.net/results/15e/15e31d92848b53f001bd6acde71409af3091215c/



[Peter: drop tvheadend hunk]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>