kernel-install: teach bot loader spec entry generator to generate entries with "tries"

kernel-install: teach bot loader spec entry generator to generate entries with "tries"

This makes two changes:

1. When called for "remove" any drop-ins with "+" suffix are removed
   too, so that the logic works for entries with boot counting enabled
   too and we don't lose track of configuration snippets created that
   way.

2. When called for "add" we optionally generate a "+" suffix, based on
   the data in /etc/kernel/tries if it exists.

   This basically means after "echo 5 > /etc/kernel/tries" any installed
   kernels will automatically set up for 5 boot tries before older
   kernels will be tried.