TFTP fixes

[OlegHahm]

In combination with RIOT-OS#4189 these fixes make Linux as a client and RIOT as a server working. Haven't tested the other way around.

[DipSwitch]

I don't realy follow because when I test it on my board with Linux as client and/or server it seems to work, altough I still have IPHC disabled as stated on the Wiki page.

Don't really like the searching trough a link list idea but if there is no other way :)
I'll test it tomorrow and merge and merge and squash it into the pull request.

Thanks for the help :)

[OlegHahm]

altough I still have IPHC disabled as stated on the Wiki page

Interesting hint. The compression could be indeed a cause for the reordering IMO. Which Wiki page are you referring to?

Don't really like the searching trough a link list idea

Well, as @authmillenon wrote in the other PR: that's the way it is done in most places in the stack and remember that this list contains almost never more than 4 or 5 elements.

[DipSwitch]

Sorry it was late, nhc_udp is disabled.