use quotes when checking empty strings in scripts

this is far less error-prone

Signed-off-by: Leah Rowe <leah@libreboot.org>

include/option.sh

@@ -41,11 +41,11 @@ handle_coreboot_utils()
 {
 	for util in cbfstool ifdtool; do
 		x_ ./update project build ${_f} "src/coreboot/${1}/util/${util}"
-		[ -z ${mode} ] && [ ! -f "cbutils/${1}/${util}" ] && \
+		[ -z "${mode}" ] && [ ! -f "cbutils/${1}/${util}" ] && \
 			x_ mkdir -p "cbutils/${1}" && \
 			x_ cp "src/coreboot/${1}/util/${util}/${util}" \
 			    "cbutils/${1}"
-		[ -z ${mode} ] || \
+		[ -z "${mode}" ] || \
 			x_ rm -Rf "cbutils/${1}"
 	done
 }