Printing a string is luxury! When I was working on a Linux port to the Treo 650 ages ago, I had a single bit of output: an LED that I could turn on or off at certain parts of the boot sequence. That was good enough to get me to the point where the LCD could be initialized and I could do proper screen logging.

I've done this for a handful of other projects as well. My first step is always locating a processor GPIO that controls something obvious (camera flash, status LED, etc) that I can use to figure out where I am in the boot process.