Disable UART on boot

[markusbkk]

As previously discussed on Telegram, modern AMD64 hardware doesn't fare too well when it comes to certain ACPI feature sets. Chief amongst them ACPI enumeration for UART (on my own Framework 13 it actually leads to a boot failure and a cryptic report about the laptop's lid state).

I suggest we disable UART on boot (at least on AMD64), as this is widely documented (1, 2) to break on newer boards.

UART can either be disabled through /boot/device.hints or /boot/loader.conf.

In both cases we'd set hint.uart.0.disabled="1".

[mszoek]

Fixed with commit 70f773d