New review comment by tornaria on void-packages repository

https://github.com/void-linux/void-packages/pull/29780#discussion_r703026578

Comment:
> BTW, there's another bug wrt bootorder: if the bootorder list has an empty slot, saving the order and then restoring like this script will change the entries after the empty slot.

Sorry, my explanation is a mess. What happens is:
 - say kernel is on entry 0005, which is the first in bootorder
 - now install a new version of the kernel
 - script removes entry for this kernel (0005)
 - script adds entry for the new version of the kernel
Result: if 0000-0004 are occupied then the new kernel will be 0005, but if one of 0000-0004 is empty, new kernel will go to the empty slot and 0005 will be empty. Then the bootorder is screwed.

The better way to do it would be:
 - script checks if there's an entry for the kernel it's replacing
 - if it is, it removes this old entry, and uses the same number for the new entry