[image] Allow opaque URI component to provide image name

[image] Allow opaque URI component to provide image name

Some URI schemes allow for a path name to be specified via the opaque
component of the URI (e.g. "file:/script.ipxe" to specify a path on
the filesystem from which iPXE itself was loaded).  Files loaded from
such paths will currently fail to be assigned an appropriate name,
since only the path component of the URI will be used to construct a
default image name.

Fix by falling back to attempt deriving an image name from the opaque
component of a URI, if no path component is specified.

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