There's no guarantee - some will ship with an updated BIOS, others won't. It's a smarter idea to assume they won't ship with a KabyLake BIOS compatible version.
What it means for you, is dropping a G4560 into a 100 series board will be pointless, as it just won't work (won't post etc).
To update it, you need a compatible chip, with the cheapest Skylake chip being the G3900 for around $40 (or find a local store that'll do it for you, probably for ~$20 or so).
Some high-end boards support BIOS updating without a CPU installed, but those are limited to very high end ($200-$250+) boards and only a couple of manufacturers. Those boards would make absolutely zero sense paired with a G4560.
The best solution would be simply to opt...