buildsystem: Drop explicit (k)config.h includes

We have kconfig.h auto-included and it pulls config.h too.

Change-Id: I665a0a168b0d4d3b8f3a27203827b542769988da
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/17655
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins)

src/commonlib/configstring.c

@@ -27,7 +27,6 @@
  * MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
  */
 
-#include <config.h>
 #include <string.h>
 #include <commonlib/configstring.h>