build: use busybox gzip compatible force option

commit 138c763 ("build: add --force option to gzip in Build/gzip")
added the --force flag to the gzip invocation.

Under environments with busybox gzip (e.g Alpine Linux), this fails
as busybox only recognizes "-f".

Signed-off-by: Mathew McBride <matt@traverse.com.au>

include/image-commands.mk

@@ -135,7 +135,7 @@ define Build/lzma-no-dict
 endef
 
 define Build/gzip
-	gzip --force -9n -c $@ $(1) > $@.new
+	gzip -f -9n -c $@ $(1) > $@.new
 	@mv $@.new $@
 endef