base-files: use "ports" array in board.json network for bridges

Bridge aggregates multiple ports so use a more accurate name ("ports")
and format (array) for storing them in board.json.

Example:

"network": {
	"lan": {
		"ports": [
			"lan1",
			"lan2",
			"lan3",
			"lan4"
		],
		"protocol": "static"
	},
	"wan": {
		"ifname": "wan",
		"protocol": "dhcp"
	}
}

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