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/mem_ops_aligned.h b/aom_ports/mem_ops_aligned.h
index 8c3ab1c..81fe41a 100644
--- a/aom_ports/mem_ops_aligned.h
+++ b/aom_ports/mem_ops_aligned.h
@@ -91,7 +91,8 @@
     *mem = (uint##sz##_t)raw;                                           \
   }
 
-#include "aom_config.h"
+#include "config/aom_config.h"
+
 #if CONFIG_BIG_ENDIAN
 #define mem_get_be_aligned_generic(sz) mem_get_ne_aligned_generic(be, sz)
 #define mem_get_sbe_aligned_generic(sz) mem_get_sne_aligned_generic(be, sz)