gptboot, zfsboot, gptzfsboot: Enable the video and serial consoles early

gptboot, zfsboot, gptzfsboot: Enable the video and serial consoles early

Normally the serial console is not enabled until /boot.config is read and
we know how the serial console should be configured.  Initialize the
consoles early in 'dual' mode (serial & keyboard) with a default serial
rate of 115200. Then serial is re-initialized once the disk is decrypted
and the /boot.config file can be read.

This allows the GELIBoot passphrase to be provided via the serial console.

PR:		221526
Requested by:	many
Reviewed by:	imp
Sponsored by:	Klara Systems
Differential Revision:	https://reviews.freebsd.org/D15862