[PATCH] fix SigSegV and hang with grub-emu-usb

[Vladimir 'phcoder' Serbinenko]

Hello when testing grub-emu with USB support I stumbled across  several problems
1) compile time warning of undefined grub_usb_libinit
2) When launched under normal user it crashed
3) When launched as superuser it hanged on ls
Here is the fix. Formatting omitted for readability
-- 
Regards
Vladimir 'phcoder' Serbinenko

usbfix.diff
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