b4 prep-perpatch-check-cmd conf via .b4-config

[Mattijs Korpershoek <mkorpershoek@baylibre.com>]

Hi,

I'm yet another (very) happy user of b4.
Thank you for developping and maintaining it.

I've noticed that v0.14+ contains the new b4 prep --check command.

This can be customized via b4.prep-perpatch-check-cmd [1]
However, we cannot customize this via a project specific .b4-config [2]
file.

It would be useful to be able to customize this via .b4-config, for
example to invoke checkpatch.pl with additional arguments (on a
per-project basis).

For example, in U-Boot, I would have the following .b4-config:
[b4]
prep-perpatch-check-cmd = ./scripts/checkpatch.pl -q --terse --no-summary --mailback --u-boot --showfile

Looking at _setup_main_config(), I can see that wtglobs does not contain
'prep-*', but adding it seems not sufficient, probably due to multivals
parsing.

I don't have enough knowledge/time to dive deeper on this, but could we
consider adding support for configuring prep-perpatch-check-cmd in
.b4-config?

Thank you!
Mattijs

[1] https://b4.docs.kernel.org/en/latest/config.html#contributor-oriented-settings
[2] https://b4.docs.kernel.org/en/latest/config.html#per-project-defaults