Thank you for your query.
In such instance, I would recommend using menuconfig , instructions bellow:
1) Navigate to your GPL folder
2) run ./scripts/feeds update -a
3) run make menuconfig
4) Navigate to: Base system -> Branding -> libbrand-versioning
5) Mark both: libbrand and libbrand-versioning with an asterisk(*)
Note: Navigate with arrow keys, and press space button to change between blank, M and *. Make sure you set * on both of these as shown bellow.
6) click Enter on libbrand-versioning a new window will show up, find RUT2Xx PREFIX and click enter.
7) Type in your custom FW prefix and hit enter.