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