If barebox is builded out-of-tree we don't find the autogenerated
autoconf.h because we search in the srctree.
With this patch we don't get this error message:
cc1: fatal error: /[...]/barebox/include/generated/autoconf.h: No such file or directory
Signed-off-by: Silvio Fricke <silvio.fricke@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>