fastboot: Make it print something useful while in this mode

Many, many times we've assumed that our boards have become unresponsive while
connected to serial, when, in fact, the board is actually just in fastboot mode.
This adds a little message that gets printed out to the serial console when the
user presses any key.

Change-Id: Iae1ed62ac73caea6b7b75d3ae9a3a6d2ea9686c6
1 file changed