x86/reboot: Use bios reboot for GIGABYTE BRIX BXBT-2807

Using ACPI, the hard disk crashes on reboot. This seems to be avoided
when using the BIOS to reboot.

[endlessm/eos-shell#3835]
Merge pull request #12 from endlessm/shell/3835

x86/reboot: Use bios reboot for GIGABYTE BRIX BXBT-2807