[SCSI] move iscsi to a better place in Kconfig

Now it looks like we'll have multiple users of the iscsi transport
class, the iscsi initiator shouldn't really be a dependency of it.  This
patch moves iscsi to being an initiator in its own right which selects
the transport attributes.

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>