The SCSI layer currently implements a retry loop in order to retry
commands that fail due to spurious "error" conditions such as "power
on occurred".  Move this retry loop to the generic SAN device layer:
this allow for retries due to other transient error conditions such as
an iSCSI target having dropped the connection due to inactivity.

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

The concept of the SAN drive number is meaningful only in a BIOS
environment, where it represents the INT13 drive number (0x80 for the
first hard disk).  We retain this concept in a UEFI environment to
allow for a simple way for iPXE commands to refer to SAN drives.

Centralise the concept of the default drive number, since it is shared
between all supported environments.

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

Create a central SAN device abstraction to be shared between BIOS and
UEFI.

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