[U-Boot] Disabling Auto Negotiation

[Arno]

In case my on-board linux is not booting (kernel/rootfs problem), I would like to have a chance to recover the device. As the only interface is ethernet, I could try a tftpboot first and only if this fails boot from flash. 
This of course has disadvantage that this tfpboot trial needs time, most with establish connection and autonegotiation. Thus question is how can I disable this to save time.

I found (http://knowledge.digi.com/articles/Knowledge_Base_Article/How-do-I-disable-Ethernet-auto-negotiation-on-the-ConnectCore-9U) that it might possible to set link. However this doesn't seems to be supported (anymore?).

I also found this thread:
http://u-boot.10912.n7.nabble.com/Disabling-Autonegotiation-td216666.html#none

which doesn't answer this, but give first ideas.

So back to the experts, how to do this best? Establish a ethernet connections is required anyhow to chose the boot option.

Best reards
Arno