It seems to be necessary for me to use the debug output using a serial cable on UART0
u-boot seems to be waiting for a Ethernet or serial cable connection to finish boot process.
I don't have the serial cable so I just need some understanding that that is the case considering that I followed the rest of the instructions on the blog at https://research.csiro.au/tsblog/sel4-raspberry-pi-3/
, newbie question btw.