export CROSS_COMPILE and test that it exists before starting

This allows us to test that the cross compiler can be found before we start.
We don't do this check in the beaglebone black or rpi because they use their own cross compilers during build.
update trimslice kernel config
update utilite kernel config
update ODROID-XU kernel config
Merge pull request #12 from steev/master

Fix some kernel configs and add a check for cross compiler