Replacing ipconfig (klibc) with a proper dhcp client (udhcp) in initramfs for better netboot experience

The currently used dhcp client in initramfs is ipconfig from the klibc library. Not only is the binary very badly documented, it also has plenty of limitations and forcefully requests a second lease from the dhcp server during a netboot. Using udhcp in initramfs seems to overcome all the limitations ipconfig has and provides a well documented and flexible dhcp client. udhcp should be included as a default client in initramfs for netbooting.