This patch adds support of vendor sub directories for dtb files.
Subdirectories can be specified in $(dts-dirs). Kbuild traverses over
all directories while building and installing dtb files. The directory
tree is also reflected in the install path.
Tested-by: NAndrew Bresticker <abrestic@chromium.org>
Signed-off-by: NRobert Richter <rrichter@cavium.com>