Let user specify NetBSD root device.
* loader/i386/bsd.c (netbsd_root): New variable.
(netbsd_opts): New option 'root'.
(NETBSD_ROOT_ARG): New macro.
(grub_netbsd_boot): Use 'netbsd_root'.
(grub_bsd_unload): Free 'netbsd_root'.
(grub_cmd_netbsd): Fill 'netbsd_root'.