As per the comment, it'll be NULL if we're not trying to boot from an iSCSI device.  On an Intel system, we'll assign it a value in ld_ib_prop() iff we detect an iBFT; i.e., if the firmware tells us we're booting from an iSCSI volume.  This routine is called from main() somewhat prior to vfs_mountroot() where the pre-root module attach that 7119 will induce is going to occur.

Oh ok, I guess I didn't read the description in redmine closely enough.