Fix handling of wifi password with special characters

We could cover the backtic case by using nowdoc instead of
heredoc, but then there may still be problems with double quotes.
Put the PBKDF2 hash instead of plaintext password in
wpa_supplicant.conf to solve such problems.

Closes #206

imagewriter.h

@@ -109,6 +109,7 @@ public:
     Q_INVOKABLE bool hasSavedCustomizationSettings();
 
     Q_INVOKABLE QString crypt(const QByteArray &password);
+    Q_INVOKABLE QString pbkdf2(const QByteArray &psk, const QByteArray &ssid);
 
 signals:
     /* We are emiting signals with QVariant as parameters because QML likes it that way */