[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>

src/include/ipxe/sanboot.h

@@ -110,6 +110,8 @@ enum san_device_flags {
 struct san_boot_config {
 	/** Boot filename (or NULL to use default) */
 	const char *filename;
+	/** Required extra filename (or NULL to ignore) */
+	const char *extra;
 	/** UUID (or NULL to ignore UUID) */
 	union uuid *uuid;
 };