dts: Add application overlay support.

This patch adds DTS application overlay support.  Applications
requiring specific device features can specify these using
board dtsi files located in the application directory.  These overlay
files are board specific.

Issue: ZEP-1745

Change-Id: I1c5f1a8a198fe543b6d52ee8c1fb31a2038a7956
Signed-off-by: Andy Gross <andy.gross@linaro.org>