kconfig: WERROR unmet symbol dependency

When KCONFIG_WERROR env variable is set treat unmet direct
symbol dependency as a terminal condition (error).

Suggested-by: Stefan Reinauer <reinauer@google.com>
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>