iwlwifi: make Makefile more concise

Also change CONFIG_IWLCORE_RFKILL to CONFIG_IWLWIFI_RFKILL to
be more consistent with other config variables.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Acked-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>