Allow net_cleanup for loader.efi

While netbooting with loader.efi on at least one arm64 platform
which uses u-boot emulating UEFI, the kernel gets corrupted, we
suspected the u-boot ethernet driver was still running.

Use netdev.dv_cleanup for efinet_dev to address this.

This in turn requires calling dev_cleanup() before bi_load() to avoid
a loader crash since bi_load() calls ExitBootServices.

Reviewed by: imp
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D51186