Couple days ago I had success with serial port communication on same device, but my success was only partial. Don't worry serial communication worked but I was not able to stop boot process by pressing a button.
You'll need to solder only three wires to use serial communication: GND (left), RX (middle), TX (right).
Do not get confused RX on source device is connected to TX on destination device and reverse.
For power source use battery or power device through usb port.
Best luck. I am really curries to see will you succeed to prevent boot by pressing a button.