[autoboot] Allow a SAN boot as a fallback if a filename boot returns

[autoboot] Allow a SAN boot as a fallback if a filename boot returns

Currently, if both a filename and root-path are present, iPXE will
hook the SAN device but will only attempt to boot from the filename.
Change this behaviour so that both are attempted.  Users who want to
avoid booting from the SAN as a fallback can do so via the existing
"skip-san-boot" setting.

This allows for seamless deployment to a SAN target using Windows
Deployment Services (and similar products).  A user simply has to
define the root-path option in DHCP and then use WDS to deploy the
system.  No further configuration should be required.

Signed-off-by: Michael Brown <mcb30@ipxe.org>