Physical port order watched from the backside of the C20i
(from left to right) is: Internet / 1 / 2 / 3 / 4

Physical Port	Switch port
WAN             0
LAN 3           1
LAN 4           2
LAN 1           3
LAN 2           4
(not used)      5
CPU             6

Signed-off-by: Walter Sonius <walterav1984@gmail.com>
[commit message/title improvements; backport to 18.06]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
(cherry picked from commit a065cd29)