olimex_a20_olinuxino_micro: initialize the network with DHCP

To allow a more complete "out-of-the-box" startup with the defconfig,
enable automatic network configuration of eth0 using DHCP.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>