When building a new disk image for a beagle bone, it can happen that it doesn't actually boot. Now, the BBB doesn't have a serial output over the USB port as the BB white had.
BBB Debug header
The BBB has a debug header that you can connect to (J1, on the inside pf P9) and the easiest way to debug for me was to hook up another BBB.
As my BBB that I used fo debuging had the OpenROV image on, it already has the UART1 activiated. If you have a vanilla BBB you might need to activate UART1
Let the Debug start
To connect the two BBB use 3 jumper cables, connect GND from you debugger BBB from P9_01 to J1_1 of the to_debug BBB.
Conect P9_24 to J1_4 and P9_26 to J1_5.
Then on the debugger BBB, execute the following command:
picocom --b 115200 --f n /dev/ttyO1
Et voila - you are now seeing (and interacting with) the BBB serial console!