Bug#983486: zipl: allow other packages to provide config snippets

Source: s390-tools
Version: 2.15.1-2

I'm one of the maintainers of kdump-tools, which has a need to manipulate
the kernel command line parameters in boot loader configurations.
Currently zipl provides no way to do this without modifying
/etc/zipl.conf directly. It would be helpful if there was e.g. an
/etc/zipl.conf.d/ interface for dropping in additional configuration.

As an example, GRUB provides an /etc/default/grub.d/ directory, which
allows us to drop in a file like this:

$ cat /etc/default/grub.d/kdump-tools.cfg 
GRUB_CMDLINE_LINUX_DEFAULT="$GRUB_CMDLINE_LINUX_DEFAULT crashkernel=384M-:128M"

One idea of how to implement this would be to do something similar to
GRUB and ship a tool (say, update-zlib) that generates a static
/etc/zlib.conf as directed by a set of shell variables (say
/etc/default/zlib). It could then process a directory of snippets (say
/etc/default/zlib.d/*) allowing other packages to tweak/override the
configuration defined by those variables.

I realize that while that this design would have an upgrade problem
for existing users, but perhaps we could provide an opt-in-only
migration path.