mkuboot.sh: Fail if mkimage is missing

on building an uImage, I get:

$ make uImage
CHK include/linux/version.h
CHK include/generated/utsrelease.h
make[1]: `include/generated/mach-types.h' is up to date.
CALL scripts/checksyscalls.sh
CHK include/generated/compile.h
Kernel: arch/arm/boot/Image is ready
SHIPPED arch/arm/boot/compressed/lib1funcs.S
AS arch/arm/boot/compressed/lib1funcs.o
LD arch/arm/boot/compressed/vmlinux
OBJCOPY arch/arm/boot/zImage
Kernel: arch/arm/boot/zImage is ready
UIMAGE arch/arm/boot/uImage
"mkimage" command not found - U-Boot images will not be built
Image arch/arm/boot/uImage is ready
$

I.e. it says: "uImage is ready" even though the uImage file doesn't
exist because mkimage is missing.

I propose the attached patch.

Signed-off-by: Roland Stigge <[e-mail]>
Signed-off-by: Michal Marek <[e-mail]>

scripts/mkuboot.sh

···
 	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