I've played a bit with pwr button, and it seems that is button can only power-on, and does nothing when system is off (does'nt power-off, and it seems that no signal is sent to the operating system). My understanding of the schematics of the bpi-r2 leaded to the same conclusion.
So, I soldered the 2 pins just next the power button. And now, my bpi-r2 automatically boots when power cord is plugged. Gotcha !