tvheadend: disable on avr32

tvheadend requires an atomic_add operation. Either you have a gcc >=
4.3 toolchain and it uses the gcc intrinsics, or it has special code
for x86, x86-64, PPC and ARM. So in the context of Buildroot, the only
really problematic architecture is avr32, which uses gcc 4.2.

Fixes http://autobuild.buildroot.org/results/f015b1888d739c383aca61609059aa7675a4da7d/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>