sysutils/u-boot-bhyve-arm64: Fix after the 2024.01 import

Because this config is not yet upstreamed (pending arm64/bhyve/vmm
actually landing in main), we have to maintain our own for the time
being.

Tweak the config to work around two problems which arose after the
latest update:

• EFI support implies SMBIOS support, which requires that we implement a finalizer, which we don't have. Since SMBIOS support isn't required, just remove it.
• Don't configure EVENT, as we don't use it, and its configuration implies that we implement a hook that runs immediately before command processing.

While here, incorporate a patch from jrtc27 which permits booting from
images formatted using an MBR.

Reviewed by: manu, jrtc27
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D43784
Differential Revision: https://reviews.freebsd.org/D43785