utilfdt_read: pass back up the length of data read

For a follow up commit, we want to be able to scan the buffer that was
returned to us.  In order to do that safely, we need to know how big
the buffer actually is, so pass that back if requested.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

tests/testutils.c

@@ -160,7 +160,7 @@ int nodename_eq(const char *s1, const char *s2)
 void *load_blob(const char *filename)
 {
 	char *blob;
-	int ret = utilfdt_read_err(filename, &blob);
+	int ret = utilfdt_read_err(filename, &blob, NULL);
 
 	if (ret)
 		CONFIG("Couldn't open blob from \"%s\": %s", filename,