Refactor code to select boot kernel

The script to find best kernel from image (used by vm
and baremetal element) is duplicated and is not in sync
with each other. Moving the code to img-functions as a
function will reduce duplicated code and make it reusable
in future.

Since img-functions is not accessible in chroot env,
kernel selection is being moved from finalise.d to
cleanup.d in the vm element.

Change-Id: I8fbccc13a2c61a5191ef9ea5d2a8302a3e43b000
Merge "Refactor code to select boot kernel"