OS Customisation: Regex to match ssh public keys

Fixes raspberrypi#956

Previously we would just trust that users would include a correct SSH public key. Unfortunately, this isn't a viable stategy.

So, let's introduce a regex to perform some light validation of the ssh-key, to avoid having boot failures.

src/OptionsPopup.qml

@@ -403,6 +403,7 @@ Window {
                     color: radioPubKeyAuthentication.checked ? "black" : "grey"
                     // textFormat: Text.PlainText
                     Layout.leftMargin: 40
+                    validator: RegularExpressionValidator { regularExpression: /^ssh-(ed25519|rsa|dss|ecdsa) AAAA(?:[A-Za-z0-9+\/]{4})*(?:[A-Za-z0-9+\/]{2}==|[A-Za-z0-9+\/]{3}=|[A-Za-z0-9+\/]{4})( [^@]+@[^@]+)?/ }
                 }
                 TextArea {
                     id: fieldPublicKey