kexec: disable on avr32

kexec does not support the avr32 architecture; this results in autobuilder
failures such as the following.

  http://autobuild.buildroot.net/results/5f5e91c3eb622bce88402a9afc8a3e7bd5007136/build-end.log



This patch disables support for the kexec package on the avr32 architecture.

Signed-off-by: Simon Dawson <spdawson@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>