only build cbutils if required

it's sometimes done unconditionally. this change
ensures that it is not repeated needlessly.

i observed otherwise that cbfstool would be
re-built from time to time, even if it was built.

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

script/update/vendor/inject

@@ -88,7 +88,9 @@ detect_board()
 build_dependencies()
 {
 	[ -d "${cbdir}" ] || x_ ./update project trees -f coreboot default
-	x_ ./update project trees -b coreboot utils default
+	if [ ! -f "${cbfstool}" ] || [ ! -f "${ifdtool}" ]; then
+		x_ ./update project trees -b coreboot utils default
+	fi
 	x_ ./update vendor download ${board}
 }