malta: use default OpenWrt network configuration

Currently malta configures the first Ethernet device as WAN interface.
If it finds a second one it will configure it as LAN.

This commit reverses it to match armvirt and x86. If there is only one
network device it will be configured as LAN device now. If we find two
network devices the 2. one will be WAN.

If no board.d network configuration is given it will be configured in
package/base-files/files/etc/board.d/99-default_network

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
[minor typos]
Signed-off-by: Paul Spooren <mail@aparcar.org>