core: add parameter to configuration-layer/make-all-packages
core: add parameter to configuration-layer/make-all-packages

New parameter `skip-layer-discovery', if non-nil then rediscovery of layers
is skipped (used essentialy for the tests).

tsonntag/dotemacs,tsonntag/dotemacs,tsonntag/dotemacs,tsonntag/dotemacs