一：recovery (key)
在无usb接入情况下，通过按压volume-up键再上电进入recovery模式。
由于loader mode的优先级高于recovery模式，故需要断开usb连接。
* rockchip_dnl_key_pressed():
*
* (1) volume-up key (default)
* (2) menu key (If no rockusb)
*
* It's possible that USB is disabled due to developer needs
* a critial size of u-boot.bin.
*
* Disabling USB makes vbus can't be detected any more, so that
* we add menu key. The events trigger are changed:
*
* - rockusb mode(actually fallback to bootrom mode):
* "volume-up pressed + vbus=1" replaced with "menu pressed"
* - recovery mode:
* "volume-up pressed + vbus=0" replaced with "volume-up pressed"
*
* At the most time, USB is enabled and this feature is not applied.
*/
