How do I measure time delay from power ON to console messages
Rather than measure the time from a power up, it would be easier to measure the time from a reset, which should be the same thing (except for the time it takes for the power supply to come up, which should be a millisecond or so).

On page 5 of the schematic is a switch, SW2 which is the reset button.  If you tie a line to PS_SRST#, it will go low during a reset (button pressed), and back high when the reset is no longer asserted.  Start timing at this point.

I am assuming the console message comes over the USB UART on the same page.  The TX line (USB_UART_TXD) should start out high.  When the first character is output, it will go low briefly.  That should be the end of your timing.

If the console message comes over some other UART, then use the TX line of that one.