[block] Allow SAN boot device to be identified by an extra filename

[block] Allow SAN boot device to be identified by an extra filename

Add an "--extra" option that can be used to specify an extra
(non-boot) filename that must exist within the booted filesystem.

Note that only files within the FAT-formatted bootable partition will
be visible to this filter.  Files within the operating system's root
disk (e.g. "/etc/redhat-release") are not generally accessible to the
firmware and so cannot be used as the existence check filter filename.

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