build: re-add initconfig to generate .config

run make initconfig to only create the .config in outdir.
The .config will be the result of the defconfig merged with the local
application config define in CONF_FILE.

Change-Id: If2a308ac3805d32cea8dd7be7a14772f22443600
Signed-off-by: Anas Nashif <anas.nashif@intel.com>