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>
(cherry picked from commit 323a028d39cd8df37b1c6a62ac04987647ae4b67)
Signed-off-by: Kevin Hilman <khilman@linaro.org>