stand: efi create eficom console device.

Fix the 'renaming kludge' that we absolutely cannot do going forward
(it's cost us days of engineering time).

console=comconsole talks to the hardware directly. This is available
only on amd64. It is not available anywhere else (and so requires
changes for people doing comconsole on aarch64)

console=eficom talks to the console via EFI protocols. It's available
on amd64, aarch64 and riscv64. It's the first port that we find, though
it can be overriden by efi_com_port (which should be set to the UID of
the serial port, not the I/O port, despite the name). devinfo -v
will give the UID to uartX mapping.

This is an incompatible change for HYPER-V on amd64. It only works with
eficom console, so you'll need to change your configuration in
loader.conf. No compatibility hack will ever be provided for this (since
it requires renamig, which the loader cannot reliably do).

It's also an incompatible change for aarch64. comconsole will need to
change to eficom. There might be a comconsole "shim" for this.

All the interlock to keep only eficom and comconsole from both attaching
have been removed.

RelNotes: Yes
Sponsored by: Netflix
Discussed with: kevans
Differential Revision: https://reviews.freebsd.org/D39982