So I got things sorted. As Vik pointed out the serial interface 2 (pa2/pa3) is routed through the virtual com interface of th eprogrammmer’s USB interface. For those of you who are using Linux. it will be /dev/ttyACM0. So a simple cat /dev/ttyACM0 should echo all printf send from the foirmware side. in Fedora 24 or earlier there is something called ModemManager that gets in your way. Things run more smoothly if you disable it. the following worked for me:
systemctl status dbus-org.freedesktop.ModemManager1.service
systemctl disable dbus-org.freedesktop.ModemManager1.service
the easiet is now to reboot so Network manager stops using the ModemManager.
So I now finally get my data send out via serial from the F401 nucleo board. Cool. But there is still one caveat: