base-files: add ucidef_set_led_usbport for full usbport support

This helper allows using usbport trigger directly. It requires usbport
compatible syntax and supports specifying multiple USB ports, e.g.:
ucidef_set_led_usbport "usb" "USB" "devicename:colour:function" "usb1-port1" "usb2-port1"

This adds a proper object to the board.json, e.g.
"usb": {
	"name": "USB",
	"type": "usbport",
	"sysfs": "devicename:colour:function",
	"ports": [
		"usb1-port1",
		"usb2-port1"
	]
}
and supports translating it into uci section.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>