Please add a command to purge old kernels

[rrthomas]

This was https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=685101

There's logic somewhere in Debian to purge old kernels, which AFAICT is magically used when you've successfully booted a new kernel…sometimes, at least; if you don't reboot, then it never runs. It would be good to be able to run it (or equivalent) manually. Basically, purge all kernels that aren't either a) the latest or b) the one you're running.

[gjwgit]

https://manpages.debian.org/stretch/byobu/purge-old-kernels.1.en.html