[PATCH] [yeeloong] held proper mountroot waits

[Vladimir 'φ-coder/phcoder' Serbinenko]

Hello. Here the yeeloong patches split into small pieces for easier
review and merge.
First patch is to held mountroot wait during usb probing. I believe this
issue isn't yeeloong-specific but became apparent on it because of
either different speed ratios between varios hardware components or
because when I first came up with it I was testing without cache at all. 

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Regards
Vladimir 'φ-coder/phcoder' Serbinenko