Revert "Remove wacky "sysbuild" filtering"

This reverts commit 2d7e106d293c7e81b4b0b333d256aef0490ea1bc.

Unfortunately some jobsets still depend on this behaviour. They could probably do something like "assert system == input.system; …" but changing them all is undesirable.

Created by  Eelco Dolstra  on November 1, 2013
P76HTFODGDSA53OZ2LQM6Y7CSCC3CXHQNZQDN4ZML6XAFFO2CFKQC
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