ramips: clean and improve MAC address setup in 02_network

This patch removes unnecessary MAC address setup statements in
ramips' 02_network by doing several optimizations:

1. For the following devices, lan_mac was set up with
   mtd_get_mac_binary although the same address was set in DTS.
   The lan_mac statement is removed in 02_network, but
   wan_mac is kept:
   - mercury,mac1200r-v2
   - phicomm,k2g
   - skylab,skw92a
   - wiznet,wizfi630a

2. For the following devices, wan_mac was set up with
   mtd_get_mac_binary although the same address was set in DTS.
   The wan_mac statement is removed in 02_network, no
   lan_mac is present:
   - buffalo,whr-g300n
   - glinet,gl-mt300n-v2
   - zyxel,keenetic-start

3. For the following device, lan_mac and wan_mac were set up
   with mtd_get_mac_binary to the same address as set in DTS.
   Both statements are removed in 02_network:
   - buffalo,whr-600d

4. For some devices, it was possible to move setup from 02_network
   to DTS by introducing previously missing mtd_mac_address:
   - buffalo,whr-1166d
   - buffalo,whr-300hp2
   - buffalo,wsr-600dhp
   - ohyeah,oy-0001
   - planex,vr500

5. For one device, mtd_mac_address was just wrong and overwritten
   by 02_network. Put the correct value in DTS and remove redundant
   statement in 02_network:
   - asus,rt-ac57u

6. For one device, MAC address defined in DTS is exchanged together
   with lan_mac/wan_mac setup in 02_network, so that cases in
   02_network can be merged:
   - phicomm,k2p

For some devices, an empty case has to be used to prevent them
from falling into the default case and have
WAN address = eth0 address + 1 set to them.

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