[block] Allow for additional SAN boot parameters alongside filename

[block] Allow for additional SAN boot parameters alongside filename

The drive specification alone does not necessarily contain enough
information to perform a SAN boot (or local disk boot) under UEFI.  If
the next-stage bootloader is installed in the EFI system partition
under a non-standard name (e.g. "\EFI\debian\grubx64.efi") then this
explicit boot filename must also be specified.

Generalise this concept to use a "SAN boot configuration parameters"
structure (currently containing only the optional explicit boot
filename), to allow for easy expansion to provide other parameters
such as the partition UUID or volume label.

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