Thanks a lot for sharing your information.

I got it to boot as it should with help of your example.

In /boot/config.txt I added the directives:
core_freq=250
dtoverlay=pi3-disable-bt
enable_uart=1

In /boot/cmdline.txt I removed
console=ttyAMA0,115200 kgdboc=ttyAMA0,115200
and I added
plymouth.ignore-serial-consoles

Furthermore, in /etc/inittab I removed (commented out) the line:
T0:23:respawn:/sbin/agetty -L ttyAMA0 115200 vt100

The computer now boots as expected. Next step is to move it to an Internet-depraved site and see if I can get it to communicate with the 3D-printer there. :-)

Thanks for your help.