Hi,
You can check the motherboard support site and open the "CPU Support List".
There should be a DATE shown for when the BIOS added support for a particular CPU. I would avoid any board that shows the update less than TWO MONTHS AGO (because there might be an older BIOS version still in stock).
*I do agree on getting the Z270 if it's about the same price, but if you're saving some money then go with the Z170 because it probably doesn't add any functionality you care about. (I know it adds another 4x PCIe lanes for another M.2 SSD slot but beyond that I don't know. not many people care).
**Also, some motherboards can update the BIOS without a CPU being present. Asus has something called "USB BIOS Flashback" which the higher priced boards have so a lot of the Z170 board probably have this feature.
OTHER:
AMD's RYZEN CPU's are coming. If you can hold on, you may want to wait and see what is offered as you should be able to get a similar CPU for less, or a better CPU for about the same price.
*Strong rumor is that $130USD will get a 4-core CPU, but I'd spend a little more for an overclocking model. In fact, a 4C/8T (hyperthreaded, AMD calls it "SMT") is what I'd get if you are building a gaming rig.
http://www.forbes.com/sites/antonyleather/2017/02/12/amd-ryzen-biggest-leak-yet-amazing-benchmarks-and-prices-starting-at-just-129/#4e7000694bb7
The "1400X" estimate is $200. It's overclockable so it should be very close to the i7-7700K in performance for over $100 less.