One thing more I just remembered

You need to enable the serial console on kernel of course. There are many ways to do this, on your own kernels you can of course hard-code it, but easier is to give the parameters on command line. What you need is "console=ttyS0,115200n8" added there.

That can be done either by
1.) flasher setting the R&D mode flags ( --enable-rd-mode --set-rd-flags=serial-console) so that NOLO will write the command line to the kernel, or
2.) if you use ubiboot, you can patch the command line similary as patching the Wayland required options on Nemo kernel:
just change the Harmattan init script tag to G_HARMATTAN_INITSCRIPT="\/sbin\/preinit_harmattan\ console\=ttyS0\,115200n8"