board: amlogic: vim3: configure serial# from ethaddr

The Khadas VIM3 and VIM3L boards, which are supported in Android via
Yukawa [1] need a serial number for usb/fastboot enumeration.

Whenever the environment does not provide a serial#, use the eth mac
address as serial#.

[1] https://source.android.com/setup/build/devices#vim3_and_vim3l_boards



Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lore.kernel.org/r/20220119142400.127342-1-mkorpershoek@baylibre.com