1. Unlock your bootloader with the Code you can receive from Motorola. Further instructions: unlocking bootloader. You have to create a Motorola account first.
2. Flash TWRP 184.108.40.206 through bootloader. Download and how-to-flash: twrp athene
3. Backup partitons in TWRP (especially the boot-partition)
4. Flash Kernel ElementalX through TWRP. Download and further information: ElementalX
5. Wipe Cache and ART-Cache through TWRP (don't know if necessary but doesn't harm)
6. Flash SuperSU v2.79 SR3 through TWRP. Download SR3-SuperSU-v2.79-SR3-20170114223742.zip: SuperSUv2.79SR3.
SuperSU v2.79 SR3 (or newer) will flash as systemless by default on Motorola devices running Android 6+. Older versions of SuperSU won't do that without creating a config file .supersu in /data containing the line
That's it and it works. SuperSU.apk is going to be installed in step 6.
Successfully tested with XT1642, XT1622 (eta82), XT1643 (Adarsh balu), XT1644 (WTF?), XT1641 (EkisDeMX).