CSST is the tool used to sign the first stage bootloader (e.g., mbmloader, X-Loader, etc.)

You can get it here (requires ti.com account). Without having the key used by Motorola, this tool would not be useful for our purposes.

Here's some information about how the bootloading process works in TI OMAP2420, which is similar to the one in the Milestone's OMAP3430.