You can set up your X2 to power on immediately when you connect power. Just below the power button are two plated holes meant for soldering wires for another parallel power button (the letters “PWR” are printed in between these two holes). You can solder a short piece of wire permanently connecting these two holes (as if you were holding down the button). With the wire in place, the X2 will power up when you switch the power on.
If you like, you can even still shut down the power circuit by driving the “off” pin high. The X2 will turn back on after you cycle the power.
-Adam
P.S. I had a similar question when (here) when Pololu released a standalone version of their pushbutton power circuit.