linux-boot: Refactor boot-system.

The --root option can now be omitted, and inferred from the root file system
declaration instead.

* gnu/build/file-systems.scm (canonicalize-device-spec): Extend to support NFS
directly, and...
* gnu/build/linux-boot.scm (boot-system): ...remove NFS special casing from
here.  Remove nested definitions for root-fs-type, root-fs-flags and
root-fs-options, and bind those inside the let* instead.  Make "--root" take
precedence over the device field string representation of the root file
system.
* doc/guix.texi (Initial RAM Disk): Document that "--root" can be left
unspecified.