[efi] Allow booting from local disks via the "sanboot" command

Extend the EFI SAN boot code to allow for booting from a local disk,
as is already possible with the BIOS SAN boot code.

There is unfortunately no direct UEFI equivalent of the BIOS drive
number.  The UEFI shell does provide numbered mappings fs0:, blk0:,
etc, but these numberings exist only while the UEFI shell is running
and are not necessarily stable between shell invocations or across
reboots.

A substantial amount of existing third-party documentation for iPXE
will suggest using "sanboot --drive 0x80" to boot from a local disk
(when no SAN drives are present), since this suggestion has been
present in the official documentation for the "sanboot" command for
almost thirteen years.  We therefore aim to ensure that this
instruction will also work for UEFI, i.e. that in a situation where
there are local disks but no SAN disks, then the first local disk will
be treated as being drive 0x80.

We therefore assign local disks the virtual drive numbers 0x80, 0x81,
etc, matching the numbering typically used in a BIOS environment.
Where a SAN disk is already occupying one of these drive numbers, the
local disks' virtual drive numbers will be incremented as necessary.
This provides a rough approximation of the equivalent functionality
under BIOS, where existing local disks' drive numbers are remapped to
make way for SAN disks.

We do not make any attempt to sort the list of local disks: the order
used for allocating virtual drive numbers will be whatever order is
returned by LocateHandle().  This will typically match the creation
order of the EFI handles, which will typically match the hardware
enumeration order of the devices, which will typically match user
expectations as to which local disk is first, second, etc.

We explicitly do not attempt to match the numbering used by the UEFI
shell (which initially sorts in increasing order of device path, but
does not renumber when new devices are added or removed).  We can
never guarantee matching this partly transient UEFI shell numbering,
so it is best not to set any expectation that it will be matched.
(Using local drive numbers starting at 0x80 helps to avoid setting up
this impossible expectation, since the UEFI shell uses local drive
numbers starting at zero.)

Since floppy disks are essentially non-existent in any plausible UEFI
system, overload "--drive 0" to mean "boot from any drive containing
the specified (or default) boot filename".

Signed-off-by: Michael Brown <mcb30@ipxe.org>

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