Detecting network loaded uboot

[Spaker, Erich]

I would like to configure uboot to default to network booting, if uboot.img was loaded from the network itself (by SPL). Otherwise booting from the drive.
I presume some information is passed from SPL to uboot. Is the source of uboot.img (network, mmc) known to uboot after it starts?

If not, I'd like to patch spl to pass this info, but I'm not sure the appropriate mechanism.
Bloblist seems appropriate, but there seemed to be some alternatives discussed here http://u-boot.10912.n7.nabble.com/passing-info-from-SPL-to-U-Boot-td361636.html.

I am working with an am335x/beaglebone black.
Bloblist is not configured in the standard config, I'm not sure if this because of incompatibility or what the constraints are on choosing an address.