kexec: disable on unsupported architectures

kexec only supports the following architectures.

  i?86
  powerpc
  powerpc64
  arm*
  s390x|s390
  sh4|sh4a|sh3|sh
  mips*
  cris
  crisv32
  ia64
  x86_64
  alpha

The Buildroot kexec package is available when an unsupported target
architecture is selected, leading to autobuild failures like the following.

  http://autobuild.buildroot.net/results/e706b08abb4fd1987fc4aa970d6be50fc92dc1fa/build-end.log



This patch makes the kexec package available only for the supported
architectures.

Signed-off-by: Simon Dawson <spdawson@gmail.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>