> Where does it fail? 
It fails on different points within the build. In my custom layer 
configuration it fails when building the u-boot-script recipe.

IMHO the problem appears due to running those recipes in parallel (for each 
multiconfig) and  some races appear
when mounting/unmounting the WORKDIR into the rootfs.

The next problem may appear if created packages of one MACHINE will 
overwritten by the same package of another MACHINE.
Packages must contain a machine specific suffix in order to fix that. 


> Which Isar revision? 
I ran those builds with the current next branch.