BLE fixes 2020-2

Just some minor changes to more rigidly define the pairing requirements and behaviour. A secure implementation still needs https://git.card10.badge.events.ccc.de/card10/firmware/-/issues/74

Documentation/how-to-build.rst

@@ -79,6 +79,12 @@ Dependencies
 
         pip3 install --user crc16
 
+or
+
+    .. code-block:: shell-session
+
+        apt install python3-crcmod
+
 or
 
     .. code-block:: shell-session