cmake: generate rtcd headers in config subdir.

Move rtcd header outputs to config subdir of config dir,
and update include sites.

Change-Id: I25c5f1808a091f3727934adc274637ebdcdcb1cf
diff --git a/test/sad_test.cc b/test/sad_test.cc
index 092a42d..5bec2c1 100644
--- a/test/sad_test.cc
+++ b/test/sad_test.cc
@@ -16,7 +16,7 @@
 #include "third_party/googletest/src/googletest/include/gtest/gtest.h"
 
 #include "config/aom_config.h"
-#include "./aom_dsp_rtcd.h"
+#include "config/aom_dsp_rtcd.h"
 
 #include "test/acm_random.h"
 #include "test/clear_system_state.h"