configs/orangepi_lite2: add WiFi support

Orangepi lite2 board has AP6356S WiFi/BT combo, but does
not have ethernet port. So it makes sense to enable wireless
networking by default:
- add broadcom wireless firmware package to image
- add basic wireless tools to image
- add rootfs overlay with proper NVRAM file for on-board AP6356S chip
- add mdev to image to enable module autoloading
- update readme.txt to test wifi

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>