Firmware configuration file location updated

[cmitu]

On RaspiOS Bookworm or later, the config.txt path is different than earlier OSes.

Ref:

• https://www.raspberrypi.com/documentation/computers/config_txt.html
• https://github.com/raspberrypi/documentation/commit/7d1efe23e7b7227108713d14cdd17e1b41fb674a (documentation PR that changed it, since the main page may be updated when RaspiOS changes).