base-files: Changed UCI variable name for GPIO value from 'default' to 'value'

This changes the UCI variable for the GPIO value from system.$cfg.default back
to system.$cfg.value as it was before the change from uci-defaults [1] to board.d.
/etc/init.d/gpio_switch [2] still expects the value to be in system.$cfg.value.

[1] https://github.com/lede-project/source/blob/d65916047b44d6d157d88d15e8e3d92555c5e6f8/package/base-files/files/lib/functions/uci-defaults.sh#L197
[2] https://github.com/lede-project/source/blob/master/package/base-files/files/etc/init.d/gpio_switch#L17



Signed-off-by: Julian Labus <julian@labus-online.de>