cmake: Output aom_config in config sub dir.

Does away with somewhat confusing usage of "./aom_config.h" in
include statements while keeping linters silent.

aom_config.asm, aom_config.c, and aom_config.h are now written to
the config sub dir.

Change-Id: I99e2422d6ca8b20b9cdf2feee83a866c273e47b0
diff --git a/aom_dsp/sad.c b/aom_dsp/sad.c
index 933b177..a2de008 100644
--- a/aom_dsp/sad.c
+++ b/aom_dsp/sad.c
@@ -11,7 +11,8 @@
 
 #include <stdlib.h>
 
-#include "./aom_config.h"
+#include "config/aom_config.h"
+
 #include "./aom_dsp_rtcd.h"
 
 #include "aom/aom_integer.h"