How to design a soft switch controlled by USB connector?
You don't need a µcontroller if you use Micro-USB sockets and cables, because they have an additional ID pin that connects to 0V when the plug is inserted. A simple transistor inverter will convert the connection to 0V into a logic high signal that you can use to turn on your converter.

This approach gives you the option of using a Micro-AB socket on your powerbank to act as the output port to your device or for connecting a power supply to charge the battery. It's the approach adopted by many devices nowadays such as tablets and smartphones.