configs/orange-pi-zero-plus2: enable WiFi support

configs/orange-pi-zero-plus2: enable WiFi support

Orange-Pi Zero Plus2 board has AP6212 WiFi/BT combo, but does
not have ethernet port. So it makes sense to enable wireless
networking by default:

- add DTS patch enabling AP6212 WiFi/BT combo support
  -- this patch is on its way to mainline Linux v4.16
- enable WiFi support in kernel config
- enable Broadcom brcmfmac kernel driver in kernel config
- add Broadcom wireless firmware package to image
- add basic wireless tools to image
- add mdev to image to enable module autoloading
- add rootfs overlay with proper NVRAM file for on-board AP6212 chip

Signed-off-by: Sergey Matyukevich <geomatsi@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>