build: Fix Device Tree file path

Debug build for msm-copper target fails due to usage
of relative path to get the Device Tree file.

This change replaces the relative path with absolute path
from top level directory in the workspace.

Change-Id: Idd39679028209cf75c327f61703e9fc51ad32d29
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>