DragonFlyBSD

Booting from the latest release ISO (dfly-x86_64-5.6.2_REL) via EFI, with the
on-CD EFI bootloader (EFI version 2.31, EFI firmware: V(rev 1.00), EFI loader,
Revision 1.1)

The EFI bootloader picks up on the presence of the serial port and automatically
writes to both the VGA adapter and the serial port. At the bootloader, set
console=comconsole and then boot.

The kernel is loaded and modules are loaded; information about this is posted to
the serial console along with the copyright messages, indicating kprintf calls
are being redirected to the comconsole OK.

At this point, due to the bug I actually wanted to file but nothing to do with
this particular mattter, the kernel panics. Nothing is emitted to either the EFI
framebuffer or the serial console (without `console=comconsole`, the panic is
printed to the EFI framebuffer).

When booting via the BIOS bootloader, the same steps successfully boot into the
installer environment fully emitting all info to the serial console. Manually
triggering a panic via `sysctl debug.panic=1` confirms that ddb output is indeed
dumped to comconsole OK, just not when booted via EFI.

The full output emitted to the serial interface prior to the "hang" (invisible
panic) is reproduced below:

```
Type '?' for a list of commands, 'help' for more detailed help.
OK set console=comconsole
OK boot
kernel text=0xb83660 data=0x134ba20 syms=[0x8+0xfc828+0x8+0xbbce5]
ehci.ko size 0xc8f8 at 0x2488000
xhci.ko size 0xcec0 at 0x2495000
Start @ 0xffffffff802a8690 ...
EFI framebuffer information:
addr, size 0xf0000000, 0x300000
dimensions 1024 x 768
stride 1024
masks 0x00ff0000, 0x0000ff00, 0x000000ff, 0xff000000
Copyright (c) 2003-2019 The DragonFly Project.
Copyright (c) 1992-2003 The FreeBSD Project.
Copyright (c) 1979, 1980, 1983, 1986, 1988, 1989, 1991, 1992, 1993, 1994
The Regents of the University of California. All rights reserved.
```

I can retry from master if there's a reason to think the situation has changed.