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_ports/arm.h b/aom_ports/arm.h
index 448a70d..a1a2ab7 100644
--- a/aom_ports/arm.h
+++ b/aom_ports/arm.h
@@ -12,7 +12,8 @@
 #ifndef AOM_PORTS_ARM_H_
 #define AOM_PORTS_ARM_H_
 #include <stdlib.h>
-#include "aom_config.h"
+
+#include "config/aom_config.h"
 
 #ifdef __cplusplus
 extern "C" {