It happens in yocto 2.0 and 2.1 that if you press the power button for 1-2 seconds and leave it the LED flashes continuously. Even if you reboot the board it will again start to flash continuously. The only fix I found to this was by reflashing the firmware. Obviously that is not the right solution but I dont know.
Pressing the power button for 2 to 7 seconds will turn the AP mode on and the LED will start blinking. To turn off the AP mode you should press the power button again for 2 to 7 seconds. If you press it for more time the Edison will enter into the low power sleep mode.