Add backup for po/Firmware.pot

lang/update-pot.sh

@@ -1,3 +1,4 @@
 #!/bin/sh
 # Extract language data in the po subdir to keep the relative paths intact
+cp -f po/Firmware.pot po/Firmware.pot.bak
 cd po && ../lang-extract.py --no-missing -s -o Firmware.pot ../../Firmware/[a-zA-Z]*.[ch]*