Support for different init managers

[mqqc]

This MR adds support to run OpenRC under different init managers instead of just sysvinit.
It renames openrc-inittab to virtual/openrc-inittab that can be provided by different packages: openrc-inittab is moved to openrc-inittab-sysvinit that provides virtual/openrc-inittab, and openrc-inittab-busybox is added that allows OpenRC to run under BusyBox init.

Finally, it adds a subpackage for OpenRC's very own init manager, openrc-init, and adds support to run OpenRC under it (by setting VIRTUAL-RUNTIME_init_manager = "openrc-init").

I tried to make the changes clear and atomic by splitting them up into separate commits, but please let me know if anything is fuzzy or non-obvious. Thanks!

[jsbronder]

Hey, thanks for all the PRs! It'll be a little bit before I get a chance to review them all, but I did have one question up front. Why did you change the target from master to langdale? Langdale's end-of-life is next month. Ignoring that, things are typically merged into the master branch of layers first, then backported as necessary (if there's interest).

I haven't reviewed everything yet, so I don't know if these PRs are backward compatible or not. If they are, no problem with pulling them back to langdale.

[mqqc]

No problem, thanks for responding. I opted to target langdale as that's the release we're currently basing our builds on, and thus it's the one I'm able to test (not all of the dependent layers in our build are compatible with mickledore yet, sadly). As such, it's more of an indication what version of OE these PRs have been developed and tested against. I'm happy with re-targeting master, but sadly not yet able to give them the same full test this way.

[mqqc]

Done. One other change I did was set the serial consoles in BusyBox's inittab to askfirst (basically respawn with init asking first on the console to press enter to activate it), since I figure that resource-constrained systems that BusyBox init can run on may not appreciate the overhead of always having a getty running if the console is not in use. This is consistent with e.g. OpenWrt.

[jsbronder]

Finished reviewing, just a couple of requests and this is good to merge.

[mqqc]

Thanks for the review, should be all in order now!

[jsbronder]

Merged to langdale and master. Thanks!