build: add --force option to gzip in Build/gzip

When using pigz, a parallel gzip implementation, the gzip step in the
image build for some targets fails, because the image filename already
has the .gz extension. This results in an emtpy image file. Fix this by
adding the --force option to gzip in the Build/gzip macro.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>
Tested-by: Martin Schleier <drahemmaps@gmx.net>

include/image-commands.mk

@@ -133,7 +133,7 @@ define Build/lzma-no-dict
 endef
 
 define Build/gzip
-	gzip -9n -c $@ $(1) > $@.new
+	gzip --force -9n -c $@ $(1) > $@.new
 	@mv $@.new $@
 endef