rauc: prepare support for locating the config in /usr or /run

[a3f]

rauc: prepare support for locating the config in /usr or /run

RAUC PR #15571 is already merged into master and extends RAUC to
look up its system.conf in /etc/rauc, /run/rauc or /usr/lib/rauc
in that order.

The RAUC recipe understandably hardcodes the assumption that the
system.conf must be supplied and that it must be located in /etc/ra

To make the recipe more useful with RAUC with #1557 merged, let's
add RAUC_CONF_DIR, which defaults to /etc/rauc, but can be set by
the user to be /usr/lib/rauc instead for hermetic-usr use cases.

This PR does not break anything for current RAUC versions, so it
should be fine to merge it independent of the update to a future
RAUC release that integrates the upstream PR.

[a3f]

This PR can't be trivially rebased onto master and my BSP still uses Kirkstone. To avoid churn, I'd like to reach agreement first whether this approach is ok before rebasing.

[ejoerns]

I am not convinced if it is worth changing this for kirkstone.
I assume no one still sitting on a kirkstone-based BSP will transition to a different config directory.

For scarthgap however, it might make sense to support it.
However, since scarthgap we also use a separate rauc-conf.bb recipe and it should be quite easy to just write a custom one that does not install the system.conf.

For the path my favorite choice would be to just propose /usr/lib/rauc/system.conf as the default in master and add a note to the migration section.
People then can still use a different path in their custom rauc-conf recipes if required.

If it makes things easier, I would, however, be open to taking the minimalistic RAUC_CONF variable approach for both master and scarthgap.

[a3f]

I am not convinced if it is worth changing this for kirkstone.

It doesn't introduce any functional change on its own, just makes it easier for users to change the default if needed.

I assume no one still sitting on a kirkstone-based BSP will transition to a different config directory.

I will :-)

[I]t should be quite easy to just write a custom one that does not install the system.conf.

I think it would still be good to have a variable that can just be overridden to get the old behavior back (if we make it the default) or to opt-in to hermetic-usr.

For the path my favorite choice would be to just propose /usr/lib/rauc/system.conf as the default in master and add a note to the migration section.

Yes, that sounds sensible for master.

If it makes things easier, I would, however, be open to taking the minimalistic RAUC_CONF variable approach for both master and scarthgap.

How about we use that minimalistic approach in all of kirkstone, scarthgap and master and additionally change master's default..?

[ejoerns]

[I]t should be quite easy to just write a custom one that does not install the system.conf.

I think it would still be good to have a variable that can just be overridden to get the old behavior back (if we make it the default) or to opt-in to hermetic-usr.

My comment was about not installing system.conf at all. I guess we won't make this the default.

If it makes things easier, I would, however, be open to taking the minimalistic RAUC_CONF variable approach for both master and scarthgap.

How about we use that minimalistic approach in all of kirkstone, scarthgap and master and additionally change master's default..?

Yes, sounds like a plan.

[a3f]

My comment was about not installing system.conf at all. I guess we won't make this the default.

Ok, I will just drop that.

Yes, sounds like a plan.

Well, the PR for the first of the bunch is now ready to merge :-)