Fail the build noisily if "mkimage" can't be found, e.g., if we forgot to add
the u-boot directory to PATH.

Index: linux-2.6.24-rc7/scripts/mkuboot.sh
===================================================================
--- linux-2.6.24-rc7.orig/scripts/mkuboot.sh
+++ linux-2.6.24-rc7/scripts/mkuboot.sh
@@ -11,7 +11,7 @@
 	if [ -z "${MKIMAGE}" ]; then
 		# Doesn't exist
 		echo '"mkimage" command not found - U-Boot images will not be built' >&2
-		exit 0;
+		exit 1;
 	fi
 fi