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/aom_dsp/loopfilter.c b/aom_dsp/loopfilter.c
index 5be3b7d..9181027 100644
--- a/aom_dsp/loopfilter.c
+++ b/aom_dsp/loopfilter.c
@@ -12,8 +12,8 @@
 #include <stdlib.h>
 
 #include "config/aom_config.h"
+#include "config/aom_dsp_rtcd.h"
 
-#include "./aom_dsp_rtcd.h"
 #include "aom_dsp/aom_dsp_common.h"
 #include "aom_ports/mem.h"