From OLPC

[edit] No serial number semi-brick

If you have an old machine (e.g. a B2) without a serial number and you upgrade it to build 542, and you get something like this:

could not activate this XO
Serial number: SHF00000000

or like this when you try to activate:

No serial number in mfg data
No serial number
Use power button to power off

Then you need to use the following workaround.

[edit] Step 1: USB boot

Take the USB key you used to upgrade your machine and mount it on another system. Edit the file /boot/olpc.fth to look like this:

\ OLPC boot script
null$ to ramdisk
boot n:\boot\vmlinuz root=mtd0 rootfstype=jffs2 console=tty0

Note that the first line starts with "backslash space"

Now power down your laptop, insert your USB key, and power up. The laptop should boot normally.

[edit] Step 2: making it permanent

Get into the development console using Alt/=, and open the terminal. Use su to get a root prompt. Then cd /boot and make a backup copy of olpc.fth. Then edit olpc.fth

su
cd /boot
cp olpc.fth olpc.fth.saved
vi olpc.fth

Change the file as in the example given above, then save and exit.

Now your laptop should boot normally.

[edit] Credits

Many thanks to Mitch Bradley for walking me through this.