Commit 7cfdd76 ("[block] Describe all SAN devices via ACPI tables")
changed the definition of the iSCSI initiator IQN in the iBFT to
represent a common initiator IQN used for all iSCSI sessions, and
attempted to calculate this common initiator IQN by fetching the
common ${initiator-iqn} setting.

This fails when no explicit ${initiator-iqn} has been specified
(i.e. when an initiator IQN has instead been constructed from either
the hostname or system UUID), and results in an empty initiator IQN in
the iBFT.

Fix by using the initiator IQN of an arbitrary iSCSI session
present in the iBFT.

Debugged-by: Tal Aloni <tal.aloni.il@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>