commit 70b0476a upstream.

'make dtbs' in a clean tree will try running the dtc before actually
building it.  Make these rules depend upon the scripts to build it.
Signed-off-by: David Brown <davidb@codeaurora.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>