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>

package/kexec/Config.in

@@ -1,6 +1,8 @@
 config BR2_PACKAGE_KEXEC
 	bool "kexec"
-	depends on !BR2_avr32
+	depends on BR2_i386 || BR2_x86_64 || BR2_arm || BR2_armeb || \
+		BR2_mips || BR2_mips64 || BR2_mipsel || BR2_mips64el || \
+		BR2_powerpc || BR2_sh4 || BR2_sh4a || BR2_sh3
 	help
 	  Kexec is a user space utiltity for loading another kernel
 	  and asking the currently running kernel to do something with it.