discover: Change parsers to emit resources rather than filenames

This change switches the parsers over to populate the resources in
discover_boot_option, rather than the string parameters in boot_option.

To do this, we need a few things:

 * Add struct resources to discover_boot_option for the boot_image,
   initrd and icon data.

 * Have the parsers populate the resources, rather than the strings.
   Currently, parsers can all use the devpath resource type.

 * Add a resolve_resource callback to parsers; this is how the device
   handler will attempt to resolve resources.

 * Change load_file to load_url, as we should be only accessing
   (resolved) resources by URLs.

This then allows us to remove the mount map, and associated lookup code,
as well as the UUID and label links to devices.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>