[gPXE] Only register bios drive without booting

[Miller, Shao]

Good day Andrey,

In regards to your request to use gPXE to register a drive but not boot
from it, the "Install direct to SAN" link from the "HowTo Guides" lands
you here[1].  That might help for understanding the "keep-san" setting
and how it can be used to maintain the iSCSI connection after control
returns back to gPXE from an unbootable MBR.

If you have a valid MBR on the SAN and no active partitions, return
should control back to gPXE.

- Shao Miller