ugps: Add option disabled

Like many other packages, an option to disable can be practical.

Signed-off-by: Bruno Randolf <br1@einfach.org>

package/utils/ugps/files/gps.config

@@ -1,3 +1,4 @@
 config gps
 	option	'tty'	'ttyACM0'
 	option	'adjust_time'	'1'
+	option	'disabled'	'1'

package/utils/ugps/files/ugps.init

@@ -13,6 +13,9 @@ service_triggers() {
 start_service() {
 	local tty="$(uci get gps.@gps[-1].tty)"
 	local atime="$(uci get gps.@gps[-1].adjust_time)"
+	local disabled="$(uci get gps.@gps[-1].disabled || echo 0)"
+
+	[ "$disabled" == "0" ] || return
 
 	[ -c "$tty" ] || {
 		tty="/dev/$tty"