kbuild: include config.mk when auto.conf is not older than .config

Since the Kconfig conversion, config.mk has been included only when
include/config/auto.conf is newer than the .config file.

It causes build error if both files have the same time-stamps.
It is actually possible because EXT* file systems have a 1s time-stamp
resolution.

The config.mk should be included when include/config/auto.conf is
*not older* than the .config file.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reported-by: Tom Rini <trini@konsulko.com>
Reported-by: York Sun <yorksun@freescale.com>
Reported-by: Stephen Warren <swarren@nvidia.com>
Reported-by: Matthew Gerlach <mgerlach@opensource.altera.com>
Tested-by: Stephen Warren <swarren@nvidia.com>