Fix resolving FQDN given by DHCP

The DHCP can specify a host and domain name for the system. live-boot fills /etc/hostname and /etc/resolv.conf with the data from the DHCP server, but resolving the fully qualified domain name (FQDN) fails, when the host is not specified in DNS:

(initramfs) hostname -f
hostname: example-host: Host name lookup failure

To make the FQDN resolvable without DNS, libnss_files.so needs to be included in the initramfs, /etc/nsswitch.conf need to prefer files over dns, and /etc/hosts needs to be filled with the data from the DHCP server.