rpi-eeprom-config/update: Exit cleanly when not run on rpi4

Some users might forcibly install this on a board that isn't an rpi4.
Exit early, and explain why the program can't be run.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>

rpi-eeprom-update

@@ -346,6 +346,7 @@ checkDependencies() {
       echo "BCM2711 detected"
    else
       # Not a BCM2711, no EEPROMs to update.
+      echo "This tool only works with a Raspberry Pi 4"
       exit ${EXIT_SUCCESS}
    fi