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

[allanjude]

Summary

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) and a serial rate of 115200.
Serial is re-initialized again once /boot.config can be read, after the disk is decrypted.

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

Test Plan

I need to test this some a few more types of real hardware

[imp]

I'd reword the commit message.
You are really enabling serial and video early...