EFI: implement EFI block io devices, stub out video code.

* The EFI block io devices are read-only, and it will scan
  all block devices reported by EFI; in theory, one can
  forego even scanning for a partitioning system
* May want to later change the EFI block io code to only return
  disks rather than individual partitions, and rely on the
  system partitioning code
* Also forced the boot menu to show for now (start.cpp)
* With these changes, it's now possible to get to the Haiku
  boot menu on a 64-bit EFI system (at least without any
  actual Haiku installations present)