Add a routine to obtain netboot parameters from the U-Boot env vars. Call

Add a routine to obtain netboot parameters from the U-Boot env vars.  Call
it from the uboot net_init() implementation.  The routine uses the standard
U-Boot env vars plus a freebsd-specific variable named "rootpath" (the
corresponding u-boot variable for that would be "bootfile" except that it
refers to ubldr, so a new name was needed to communicate the path to ubldr).

This allows ubldr to load a kernel from nfs without requiring a dhcp or
bootp server to provide the server ip and rootpath parameters.