buildscript: fix checksum generation for *.img.gz binaries

The new x86_64 target produces gzip compressed binaries, so it is
necessary to generate checksums for *.img.gz files as well.

Signed-off-by: Fabian Bläse <fabian@blaese.de>
Reviewed-by: Christian Dresel <freifunk@dresel.systems>

buildscript

@@ -312,7 +312,7 @@ buildrelease() {
     fi
 
     cd bin/$variant
-    for binary in *.bin *.img *.tar; do
+    for binary in *.bin *.img *.img.gz *.tar; do
         md5sum "$binary" > ./"$binary".md5
         sha256sum "$binary" > ./"$binary".sha256
     done