OMG… I cant believe that Microsoft (the most power software company on the planet) and Epic… With out a doubt one of the best and complete game engines in world.
And with both of those POWER houses… We still have this CRAZY LONG UWP external build process…
IF YOU CAN GET IT TO BUILD AT ALL… I sure the XXX cant 
After TWO HOURS of building, I get a failure towards the end:
UBT Error: Failed to build UE4Editor-UWPDeviceDetector
Anybody got any clues about that one ?
UPDATE: If you have problems with UWP-DeviceDetector or WindowsDevicePortal. After you generate the Project files (using -2017). Manually add the windows device portal nugget package to the UWP.Automation project and build that first. Then build the whole UE4 solution.
That FINALLY worked for me… After many hours…
I know everyone says this… But man UWP need to be binary marketplace plugin already 