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 <stigge@antcom.de>
Signed-off-by: Michal Marek <mmarek@suse.cz>