samples: wifi: nxp: separate wifi example user config file

Separate wifi example user config file to wifi network configurations
and hostap configurations.
User can set EXTRA_CONF_FILE as overlay-hostap.conf file to get
external hostap support.
Without overlay-hostap.conf, we will have embedded hostap.

Signed-off-by: Fengming Ye <frank.ye@nxp.com>