This is just a little advanced trick that may be useful to recover a unit during development. When this is probably most useful is when there is an error in startup scripts which does not allow you to access the login screen. Instead of reflashing this method allows you to start up your unit under a minimal safe mode and possibly
To do so, with a serial cable connected watch for the following lines.

zImage starting: loaded at 0x00400000 (sp: 0x0fb49a30)
Allocating 0x3ada80 bytes for kernel ...
gunzipping (0x00000000 <- 0x0040e000:0x007b4a1c)...done 0x38e8dc bytes
Using loader supplied ramdisk at 0xdfb7000-0xfb483d7
initrd head: 0x1f8b0808
Linux/PowerPC load: root=/dev/ram rw ramdisk_size=0x15F90 ip=deflt:
:deflt: panic=1 console=ttyS0,115200

At this point press <space> then enter 'init=/bin/sh' (ignoring the quotes) and then press enter.

This will continue to boot but provide a basic shell in ramdisk with busybox tools but no persistent partitions mounted.