[autoboot] Connect SAN disk during a filename boot, if applicable

[autoboot] Connect SAN disk during a filename boot, if applicable

For performing installations direct to a SAN target, it can be very
useful to hook a SAN disk and then proceed to perform a filename boot.
For example, the user may wish to hook the (empty) SAN installation
disk and then boot into the OS installer via TFTP.  This provides an
alternative mechanism to using "keep-san" and relying on the BIOS to
fall through to boot from the installation media, which is unreliable
on many BIOSes.

When a root-path is specified in addition to a boot filename, attempt
to hook the root-path as a SAN disk before booting from the specified
filename.  Since the root-path may be used for non-SAN purposes
(e.g. an NFS root mount point), ignore the root-path if it contains a
URI scheme that we do not support.

Originally-implemented-by: Jarrod Johnson <jarrod.b.johnson@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>