Replaces "PYB: soft reboot" with "MPY: soft reboot", etc.
Having a consistent prefix across ports reduces the difference between
ports, which is a general goal. And this change won't break pyboard.py
because that tool only looks for "soft reboot".
This patch allows you to stop auto-indent by pressing enter on a second
blank line. Easier than having to use backspace, and prevents new users
from getting stuck in auto-indent mode.
