[block] Allow SAN retry count to be reconfigured

[block] Allow SAN retry count to be reconfigured

Allow the SAN retry count to be configured via the ${san-retry}
setting, defaulting to the current value of 10 retries if not
specified.

Note that setting a retry count of zero is inadvisable, since iSCSI
targets in particular will often report spurious errors such as "power
on occurred" for the first few commands.

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