Block Pi 3 USB connector in electronics drawer

As noted in #356 (closed), power should only be accepted into the Sangaboard, not into the Pi as well. Exposing both the Pi power socket and the Sangaboard power socket causes confusion.

This MR provides a break-out blanking plate on the electronics_drawer for the Pi 3 micro-USB in the same manner and the Pi 4 USB-C.

The library key "block_usbc" is changed to "block_usb". The key seems to be used only in this one file so should not affect anything else.

Closes #356 (closed)