revert r354714 "Boot arm64 kernel using booti command from U-boot."

After discussing with mmel@, it was clear this is insufficient to address
all the needs. mmel@ will commit his original patch, from
https://reviews.freebsd.org/D13861, and the additions needed from r354714
will be made afterward.

Requested by: mmel
Sponsored by: Juniper Networks, Inc.