kbuild: Create directory for target DTB

When building specific DTBs out of the kernel tree the vendor subdirs
(boot/dts/<vendor>) are not created, ensure that they are before
building the DTB.

Signed-off-by: Nathan Rossi <nathan.rossi@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Olof Johansson <olof@lixom.net>