FEATURE REQUEST: low priority build/weekly built images

[Ansuel]

The request is simple. Is it possible to implement some kind of logic to flag some target/subtarget to be built with very low priority or picked only weekly?

One example of these are "debugging" images like the malta target or even "uml" (that we currently don't build)

Would be ideal to build these image only if the buildbot is ""free"" or schedule them weekly.

We currently workaround this problem by flagging some subtarget as source-only to remove load from buildbot.

Is it possible? Would it result in lots of additional code to implement this?

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@ynezz @aparcar @dangowrt Spamming user that might be interested in this.