kconfig: write Kconfig files to autoconf.cmd in order

Currently, include/config/autoconf.cmd saves included Kconfig files in
reverse order. While this is not a big deal, it is inconsistent with
other *.cmd files generated by fixdep.

Output the included Kconfig files in the included order.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>