Bury legacy boot

[zeha]

Continuation from #50:

I'd very much appreciate this if we could this done :)

My plan would be, I think, this:

• always setup UEFI boot
• have a new --biosboot (or --legacy-boot) option, default off, which replaces all of skip-mbr, grub, syslinux
• for this code path, only available on amd64/i386, install syslinux

I'm not entirely sure what the UEFI boot requirements are partitioning- and otherwise. Maybe that needs a bit of checking.

[mika]

I'm afraid many users out there still depend on BIOS boot, so supporting BIOS and EFI at the same time would be preferable, IMO, at least for now?