With the scripts now supporting installation of multiple kernels, the assumption of 20MB being enough for the partition can easily be wrong. Update the scripts to check and use the size of what's being installed.
Fixed here: https://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?h=sumo&id=21586e595ac0856fe71e214fb6b49f3bf053b261