Speed up the boot time of usb device

As you know it takes a while to initialize usb device, and it's also depends on the usb controller chip.

So I make a mix test recently with the following method:

• built-in usb/hcd/scsi support
  
• modprobe kernel module by script
  
• use udevd
  
• replace udevd with busybox/mdev
  
• manually create device node
  
• use udevtrigger --subsystem-match param
  
• send uevent manually to /sys/bus/*/devieces/

The best result so far is around 10 seconds, including initialize a usb device, create device node and mount rootfs.



The next performance bottleneck to be fix before xPUD 0.6 release is the firefox start time, but it looks like I/O bound, let's see what can we do for it. :-)