dtc: Implement -d option to write out a dependency file

This will allow callers to rebuild .dtb files when any of the /include/d
.dtsi files are modified, not just the top-level .dts file.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>
diff --git a/scripts/dtc/srcpos.c b/scripts/dtc/srcpos.c
index 2dbc874..36a38e9 100644
--- a/scripts/dtc/srcpos.c
+++ b/scripts/dtc/srcpos.c
@@ -40,6 +40,7 @@
 	return NULL;
 }
 
+FILE *depfile; /* = NULL */
 struct srcfile_state *current_srcfile; /* = NULL */
 
 /* Detect infinite include recursion. */
@@ -67,6 +68,9 @@
 			    strerror(errno));
 	}
 
+	if (depfile)
+		fprintf(depfile, " %s", fullname);
+
 	if (fullnamep)
 		*fullnamep = fullname;
 	else