Retry up to 20 times if mounting the root device fails.  This fixes booting
from usb-storage devices, which no longer make their partitions immediately
available.

This should allow booting from root=/dev/sda1 and root=8:1 style parameters, 
whilst not breaking booting from RAID or initrd :)
I have also cleaned up the mount_block_root() function a bit.

Based on an earlier patch from William Park <opengeometry@yahoo.ca>
Replaces the existing waiting-10s-before-mounting-root-filesystem.patch patch 
in 2.6.11-rc1-mm1

Signed-off-by: Daniel Drake <dsd@gentoo.org>