The build layering feature is coming soon. 

Code reviews have been posted ...

   https://review.opendev.org/#/c/700819/2

   https://review.opendev.org/#/c/700821/9

   https://review.opendev.org/#/c/681821/28

   https://review.opendev.org/#/c/705092/

Documentation can be found in the fourth review, here ....

   https://review.opendev.org/gitweb?p=starlingx/docs.git;a=blob;f=doc/source/developer_resources/Layered_Build.rst;h=6f945d8d33f70bd91594033fcfa6abaf6271056d;hb=2a6abc988c9a52321982015831b12819586546df

Want to try it out before it merges? 

I've set up a modified manifest that pulls in the required updates ....

One of ...

   repo init -u https://opendev.org/starlingx/manifest.git -b refs/changes/04/705104/2 -m compile.xml

   repo init -u https://opendev.org/starlingx/manifest.git -b refs/changes/04/705104/2 -m distro.xml

   repo init -u https://opendev.org/starlingx/manifest.git -b refs/changes/04/705104/2 -m flock.xml

followed by:

   repo sync

Scott