task: Let `WriteBoot64` handle BIOS\MBR setups

[nuc1eon]

I was surprised to find out that our Writeboot64 script does not include a case for MBR/BIOS setups. Only UEFI cases are handled atm. That's quite an oversight...

We should add a case for handling MBR setups as well. It doesn't have to be as elaborate as the UEFI code (which supports encrypted disk setups etc...) but at least we need the basic functionality.

This should eventually fix #130.