Initial MicroPython BLE support (GATTS)

This implements the first step towards MicroPython BLE support.

It supports to:

• Read and set (temporarily) the device name
• Read BLE MAC address
• Close a connection and start advertising
• Get notified if a connection is opened or closed
• Dynamically define new services
• Read and write attributes on the local ATT server
• Notify and indicate to the peer ATT client

It does not support to:

• Get notified if an indication was acknowledged
• Modify the advertising data

Contributes to #8 (closed)