Add default and static network configuration

Reviewed By: dustin

Took care of a couple outstanding TODO items regarding ONIE network
configuration:

1. If the kernel command line (passed from u-boot) contains a
ip=.... line do not attempt DHCP configuration. This will be the
mechanism to set a static IP address for ONIE installs.

For example on a clean system a user could do this from the U-Boot
prompt:

-> setenv onie_debugargs ip=10.0.1.200:::255.255.255.0
-> boot

The syntax of the ip= parameter comes from the kernel documentation:

   linux/Documentation/filesystems/nfs/nfsroot.txt

   ip=<client-ip>:<server-ip>:<gw-ip>:<netmask>:<hostname>:<device>:<autoconf>

2. When the static IP is not set and DHCP fails fall back to a well
known (well documented) default IP address. Since there can be more
than one ethmgmt interface number the interfaces sequentially.

The default assignments goes like:

  eth0:  192.168.3.10 / 255.255.255.0
  eth1:  192.168.3.11 / 255.255.255.0
  eth2:  192.168.3.12 / 255.255.255.0
  ...