When timesync fails, retry wifi association
Only use hexadecimal passphrases as WEP keys

Previously, every 10 or 26-character passphrase would be set as a WEP
key. However, if it would not contain only hexadecimal digits, the
gainspan module would return an error, which would then also prevent the
rest of the configuration from being saved.

In practice, this would break most 10 or 26-character WPA passphrases
(unless they only contained hexadecimal digits).

This fixes #96.