[dt] Allow for creation of standalone devices

[dt] Allow for creation of standalone devices

We will want to be able to create the console device as early as
possible.  Refactor devicetree probing to remove the assumption that a
devicetree device must have a devicetree parent, and expose functions
to allow a standalone device to be created given only the offset of a
node within the tree.

The full device path is no longer trivial to construct with this
assumption removed.  The full path is currently used only for debug
messages.  Remove the stored full path, use just the node name for
debug messages, and ensure that the topology information previously
visible in the full path is reconstructible from the combined debug
output if needed.

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