ramips: tidy up MAC address setup for Linkit Smart and Omega2

ramips: tidy up MAC address setup for Linkit Smart and Omega2

Linkit Smart 7688 and Onion Omega 2(+) are one-port devices, and
have their port set to LAN by default. Setting up a WAN MAC address
for them doesn't make any sense, as no wan interface will be created
in uci config. Despite, these devices also set lan_mac in 02_network,
although mtd-mac-address sets a different address for the ethernet
interface in DTS.

Clean this up by moving the lan_mac value into DTS and dropping the
entries in 02_network completely. That way, the effective address
on the LAN interface should stay the same, but we get rid of the
extra (re)assignments.

As I don't have access to the devices, this does not tell anything
about whether 0x2e is actually a good choice, it just preserves
the existing assignment.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>