Generate derived entries

[Damiano Scalise]

I sometime need to boot some Linux OS with optional kernel parameters which
are long to write and prone to typos.

I don't always use the same combination of parameters, and sometimes I need
none. For now I'd be happy to use three different sets of parameters for
each OS entry.

I could write static custom entries in 40_custom or custom.cfg, but then
I'd have to correct them every time something in the original entries
changes, such a kernel update.

Is there a better way to do this? Like generating the derived entries
automatically/dynamically?
Ideally I'd want a custom entry that just appends some kernel parameters,
and another custom entry which changes the field of the root= parameter.
Unfortunately I'm am not an expert at scripting myself. Thanks for any help.