ramips: Cudy X6 fixes / improvements

ramips: Cudy X6 fixes / improvements

- Correct WiFi MACs, they didn't match oem firmware
- Move nvmem-cells to bdinfo partition and remove &bdinfo reference
- Add OEM device model name R13 to SUPPORTED_DEVICES
  This allows sysupgrading from Cudy's OpenWrt fork without force
- Label red_led and use it during failsafe mode and upgrades

MAC addresses as verified by OEM firmware:

use   address             source
LAN   b4:4b:d6:2d:c8:4a   label
WAN   b4:4b:d6:2d:c8:4b   label + 1
2g    b4:4b:d6:2d:c8:4a   label
5g    b6:4b:d6:3d:c8:4a   label + LA-Bit set + 4th oktet increased

The label MAC address is found in bdinfo 0xde00.

Signed-off-by: Felix Baumann <felix.bau@gmx.de>
[read wifi mac from flash offset]
Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit 45cf200b2e22c34f2ae043b87e24230de526fefc)