uefi stand: Guess the console better

For server machines, ComOut is set to the set of devices that the efi
console suppots. Parse it to see if we have serial, video or both.
Make that take precidence over the command line args. boot1.efi parses
them, but loader.efi doesn't. It's not clear where to read boot.conf
from, so we don't do that. The command line args can still be set via
efibootmgr, which is more inline with the UEFI boot manager to replace
that. These args are typically used only to set serial vs video and
the com speed line. We can infer that from ComOut, so do so.
Remember the com speed and hw.uart.console to match.

RelNotes: yes
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D15917