AMD's AM4 should be compatible with Zen2 but Intel's current 1151 socket boards will not support CoffeeLake.
Don't buy Ryzen and an AM4 motherboard just because of that though.
- Zen 2 might require AM4+
- If AM4 ends up being compatible an old AM4 board might or might not get the BIOS update it needs
- Zen 2 might or might not be a better CPU than whatever intel is offering at that time.
If Ryzen and Coffee Lake are roughly equal for your needs, then using AM4 possible future proofing makes sense as a tie-breaker.
My philosophy: Decide which CPU meets your needs now and for the next few years. Pick a decent motherboard for it. Use it for years. By the time I've upgraded in the past I've wanted a new motherboard for newer USB, SATA, faster DRAM etc. anyway.