Allow efi loader to get network params from uboot

Summary:
efi loader does not work with static network parameters. It always uses
BOOTP/DHCP and also uses RARP as a fallback. Problems with DHCP servers can
cause the loader to fail to populate network parameters.

Submitted by: Siddharth Tuli <siddharthtuli_gmail.com>
Reviewed by: imp
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D20811