echoplayer: implement basic power-on / power-off logic

Use cpu_power_on as a GPIO to keep the main power supply
enabled when running from battery.

Change-Id: Ic79283a0cc640585a0297e11f9dcc2264ec9af26

bootloader/echoplayer.c

@@ -119,6 +119,10 @@ void main(void)
                 demo_page -= 1;
             break;
 
+        case BUTTON_X:
+            power_off();
+            break;
+
         default:
             break;
         }