filesystem is discovered. Preference is given to using the kernel
environment variable vfs.root.mountfrom, which is set by the loader
according to the contents of /etc/fstab. Changes in the MD code
provide fallback mechanisms for systems not using the loader.
A more robust fallback path is also provided, with the last recourse
being to prompt on the console for a root device.
These changes drastically simplify the machine-dependant parts of
the root configuration process. In addition, support for CDROM root
devices has been removed; it was a nasty hack and didn't work.
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