rockchip: use stable MAC-address for NanoPi R2S

rockchip: use stable MAC-address for NanoPi R2S

The NanoPi R2S does not have a board specific MAC address written inside
e.g. an EEPROM, hence why it is randomly generated on first boot.

The issue with that however is the lack of a driver for the PRNG.
It often results to the same MAC address used on multiple boards by
default, as urngd is not active at this early stage resulting in low
available entropy.

There is however a semi-unique identifier available to us, which is the
CID of the used SD card. It is unique to each SD card, hence we can use
it to generate the MAC address used for LAN and WAN.

Signed-off-by: David Bauer <mail@david-bauer.net>