tools/env/fw_env.h: Correct include order

tools/env/fw_env.h: Correct include order

When building tools-only (or env) we need to be sure that we do use
<linux/kconfig.h> and do not use <generated/autoconf.h>.  This will fix
problems such as running 'make defconfig' or 'make sandbox_config' and
then 'make tools-only'.

Based on the responses below to the thread add linux/kconfig.h higher in
the includes and drop the now unneeded autoconf.h lower down to ensure
the default environment is included correctly

http://lists.denx.de/pipermail/u-boot/2015-June/216849.html

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>