image-commands.mk: Be consistent in command invocation

Most/all other tools use the staging dir prefix, gzip should as well.

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>
Acked-by: Christian Marangi <ansuelsmth@gmail.com>
(cherry picked from commit 47df168)

include/image-commands.mk

@@ -298,7 +298,7 @@ define Build/fit
 endef
 
 define Build/gzip
-	gzip -f -9n -c $@ $(1) > $@.new
+	$(STAGING_DIR_HOST)/bin/gzip -f -9n -c $@ $(1) > $@.new
 	@mv $@.new $@
 endef