Implementing AES MixColumns with fewest XOR gates
many thanks to everyone. When I was reading originally the FIPS 197 document I made one big mistake: I assumed that the appendix C had only the cipher portion, similar to the appendix B, and missed the uncipher portions.  

Answering my own question, yes, translation of the variable temp to the one I proposed initially was correct. However, my error come from another part: For ciphering, you can save directly the results in the state array, but for the decryption you need 3 auxiliary variables more. 

I'll clean the code a little more and try to suggest an update to the Contiki repository with my part.  

Again, many thanks to everyone by guiding this misled soul