dts, kbuild: Implement support for dtb vendor subdirs

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: Andrew Bresticker <abrestic@chromium.org>
Signed-off-by: Robert Richter <rrichter@cavium.com>