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

aom_dsp/bitreader_buffer.c

diff --git a/aom_dsp/bitreader_buffer.c b/aom_dsp/bitreader_buffer.c
index bd3d0ab..c0055c9 100644
--- a/aom_dsp/bitreader_buffer.c
+++ b/aom_dsp/bitreader_buffer.c
@@ -8,7 +8,8 @@
  * Media Patent License 1.0 was not distributed with this source code in the
  * PATENTS file, you can obtain it at www.aomedia.org/license/patent.
  */
-#include "./aom_config.h"
+#include "config/aom_config.h"
+
 #include "./bitreader_buffer.h"
 
 size_t aom_rb_bytes_read(struct aom_read_bit_buffer *rb) {