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/av1/common/filter.h b/av1/common/filter.h
index 9019180..e576949 100644
--- a/av1/common/filter.h
+++ b/av1/common/filter.h
@@ -14,7 +14,8 @@
 
 #include <assert.h>
 
-#include "./aom_config.h"
+#include "config/aom_config.h"
+
 #include "aom/aom_integer.h"
 #include "aom_dsp/aom_filter.h"
 #include "aom_ports/mem.h"