[autoboot] Introduce "skip-san-boot" option

[autoboot] Introduce "skip-san-boot" option

For some install-to-SAN scenarios, the OS needs to be able to reboot
to reread the partition table.  On this second boot attempt, the SAN
disk will not be empty and so iPXE will attempt to boot from it,
rather than falling back to the OS' installation media.

Work around this problem by introducing the "skip-san-boot" option,
similar in spirit to "keep-san".

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