Hello,
FYI I am able to build a system using Yocto 'scarthgap' and Qt5 actually using the standard 'meta-qt5' layer.
I wonder where your 'meta-qt5-5-12' comes from.
 
Just my two cents.
--
Marco Cavallini
KOAN - Abinsula Group
Bergamo, Italia
https://KoanSoftware.com