Use better examples for MadWifi path in defconfig, clarify comments

Use better examples for MadWifi path in defconfig, clarify comments

MadWifi is unlikely to be in ../head relative to hostapd or
wpa_supplicant, as it would be inside the hostap git repository.
MadWifi sources are more likely to be in a directory called "madwifi"
and residing outside the hostap repository.  Using "madwifi" also
demonstrates that the top-level madwifi directory is needed.