buildbot: next steps

[pktpls]

With #63 we now have working snapshot/1.3.0-snapshot/1.2.3-snapshot builds for both targets and packages.

Important next steps:

• Publish releases
• Build sollen nur grün sein wenn alle Subbuilds grün sind
• Fix Nightlies, somehow they get confused between repos and sometimes try to build from falter-packages.git
• Enable a delay of a few minutes before webhook-triggered builds start
• Build fewer targets for snapshots
• Remove the targets build container timeout, builds take longer too often. Build cancellation is reliable so we can always cancel long-running builds manually.
• Janitor in Buildbot that cleans up old builds, so we don't run out of disk space (logs and artifacts).
• Smarter snapshot publishing. Right now we just throw the previous build away. Instead:
  • Keep previous versions of packages as discussed in build.sh: building with stable-sdk falter-packages#344
  • Keep previous builds of specific targets where the next build has failed, so that there's always images available even if the build for that target is currently failing.
  • And don't publish if the build was cancelled
• Fix Builter silent errors

Other next steps:

• Change PeriodicSchedulers to NightlySchedulers, so that they run at fixed times.
• Keep logs for targets builds, just as each package architecture has a build.log file.
• Create version stamps, downloads.openwrt.org has feeds.conf with package feeds versions, and (feeds|config|version).buildinfo for targets.

[pktpls]

Btw I keep the checklist here updated regularly :)