Design piggyback for v3 PCB

[matthijskooijman]

This piggyback could be used for:

• Attaching a solar panel (or other energy collection device) to charge a Lipo or regular rechargeable batteries
• Provide a step-up to 5V from a battery/LiPo voltage
• Provide access to additional or more complex sensors

[matthijskooijman]

For compatibility with the existing v1/v2 PCBs, the piggyback should connect to the FTDI serial connector and the power select header. For v3 boards an additional header can provide more connections (and more stability by putting the header opposite the existing header). For v1/v2 boards, extra connections and stability could perhaps be obtained through the protoboard.

The piggy-back should certainly have access to some datapins. I2c and ISP are obvious, since they can be easily shared. This allows putting an MCU on the piggyback for maintainance tasks, such as reading additional sensors, enabling/disabling a step-up converter, controlling or querying a solar/battery charger, etc. I2c is probably the easiest for this, but it might be good to attach SPI as well as some GPIO pins.

See also https://github.com/meetjestad/mjs_pcb/issues/2#issuecomment-447341849