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

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 <[email protected]>
Reported-by: Tom Rini <[email protected]>
Reported-by: York Sun <[email protected]>
Reported-by: Stephen Warren <[email protected]>
Reported-by: Matthew Gerlach <[email protected]>
Tested-by: Stephen Warren <[email protected]>