kexec: fix build failure

Add patch retrived from upstream and fixing undefined bzImage_support_efi_boot
build failure.

Fixes:
  http://autobuild.buildroot.org/results/c35/c3525dd25a37db7a976c7019901160e449363b5d/build-end.log



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>