u-boot on Edison expects vmlinuz (not vmlinux) to be present in the vfat boot partition (partition 7). NOT bzImage in the /boot folder in the ext4 partition, which is entiretly redundant. Why that's still there I don't know. That one caught me out for quite a while...
So if you bitbake your new kernel, look for vmlinuz (it'll be in there somewhere under build/tmp in the Yocto environment), and copy that to the vfat partition (mounted at /boot in latest ubilinux), reboot, and it should use vmlinuz. It should be about 5-7MB, maybe a bit bigger, if you've added lots of stuff.
Dave.