If the blspec entry does not specify a devicetree to test against,
it doesn't make any sense to check the compatible of the machine or
find the root node.

Instead of first testing the barebox devicetree check if the entry
specifies one.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>