Add dhcp support for ramdisk element.

This change adds support for retrieving the IP
through DHCP if 'ip' was not provided as a kernel
command-line argument by pxelinux.  This is used
when the kernel/ramdisk is booted by virtual media.

Change-Id: I1097ce5b56ad40f2d6dc3181681d54f924ec7145
Closes-Bug: #1321563