kbuild/mkuboot.sh: allow spaces in CROSS_COMPILE

I'm currently using CROSS_COMPILE="ccache arm-linux-".  With that the bash
builtin command "type" searches for ccache and arm-linux-mkimage and so sets
MKIMAGE="/path/to/ccache" as I don't have arm-linux-mkimage.  Then the script
dies with an error, that ccache doesn't support the argument -A.

This patch adds some quoting such that it works again for me.

Please note that this patch doesn't help you if you use ${CROSSCOMPILE}-mkimage
and ccache as mkuboot.sh now searches for the command
"ccache arm-linux-mkimage".
Signed-off-by: Uwe Kleine-König <ukleinek@informatik.uni-freiburg.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

scripts/mkuboot.sh

@@ -4,7 +4,7 @@
 # Build U-Boot image when `mkimage' tool is available.
 #
 
-MKIMAGE=$(type -path ${CROSS_COMPILE}mkimage)
+MKIMAGE=$(type -path "${CROSS_COMPILE}mkimage")
 
 if [ -z "${MKIMAGE}" ]; then
 	MKIMAGE=$(type -path mkimage)