fw_env.h: include autoconf.h

Without this, when CONFIG_ENV_VARS_UBOOT_CONFIG is active we get
a compile time error when doing 'make env'.
In file included from tools/env/fw_env.c:117:0:
include/env_default.h:110:11: error: expected ‘}’ before ‘CONFIG_SYS_ARCH’

When building U-Boot this is included indirectly by the compiler switch
-include
/home/trdx/git.toradex.com/u-boot-2014.10-toradex/include/linux/kconfig.h

Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>