build: Add separate DTS target

This patch adds an actual dts target for builds.  This required moving the
rules from Kbuild to Makefile and adding the correct dependencies to targets
requiring the dts files.

JIRA: ZEP-1979

Change-Id: I6ad568e2a821630911623cb227aa303fbf388515
Signed-off-by: Andy Gross <andy.gross@linaro.org>