Fix latest kernel+initrd detection logic

The linux-base package has a linux-version program, which does exactly what we want and need, so use that. Thanks to the following commit: u-boot-menu@4e40d9bf

For the detection of the latest initrd, the same manual parsing was done, which had the same problem, but moreover, it could find an initrd which did not match the corresponding kernel, which is bad. So change the logic to only find the initrd corresponding to the kernel image found just earlier.

Closes: #979977

Also improved code readability a bit by simplifying it and making it more explicit.