linux-headers cross compiles correctly in the next release of linux-linaro-*.
There are two remaining problems. for the first I have added a bug:
Cross compiling requires turning off do_tools: https://bugs.launchpad.net/ubuntu/+source/linux-linaro-mx51/+bug/737600
The second issue is that building the kernel package is hard for anyone not accustomed to ubuntu kernel packaging. I have started a wiki page but this still a WIP.