Fix for WiFi networks secured with WEP

This fixes a problem when configuring WiFi via serial when the access point is secured with a WEP key.

The problem was that wifi_add_profile_callback() in src/spark_wlan.cpp was not passing the correct arguments to wlan_add_profile() when the security_type was WEP. The WEP key needed to be converted from a human-readable string to a byte array.

This is only a basic fix and probably needs some improvements. It only works with hexadecimal WEP keys, and not WEP pass-phrases.