I did an installation from the ISO-build that was dd'ed to an USB-stick. Brilliant, how simple the taking off has become! I copied kernel and initRD to the hard-disk and installed the boot-loader. When upgrading I realized that the ISO-build uses a different naming convention than the regular builds (missing dash between object and version name, e.g. dCorexenial instead of dCore-xenial).
I do not see a specific reason for this and recognized the following pitfall: upon upgrading, I had to adapt the boot loader configuration; otherwise the system would not have booted. I suggest using the same naming convention.