kconfig: Use KCONFIG_CONFIG instead of .config

When using a custom location for kernel config files this merge config
command fails as it doesn't use the configuration set with
KCONFIG_CONFIG.

Signed-off-by: Markus Schneider-Pargmann <msp@baylibre.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>