The PPCBOOT link was interesting, but slightly off-topic for gdbstubs.
Have you looked into Red Boot, the bootloader that ships with eCos
(Red Hat RTOS)? It can do TFTP boots, can manage multiple images,
supports scripting, etc.; in short, it looks like it can do everything
your PPCBOOT stuff intends to do. And it is shipping today, just
download the latest eCos disto from http://sources.redhat.com/ecos and
look in the "redboot" directory.
Red Boot isn't eCos-specific, but it is using eCos's hardware
abstraction layer code to get compatability with nearly every target
that eCos supports. I have personally seen Red Boot launch both eCos
and Linux (not at the same time, of course) on an Assabet (SA1110)
board, and I was very impressed. And I don't impress easily, ask
anyone who knows me.
I also spent a few minutes with Red Boot's lead developer, and he
clearly knows his shit backwards *and* forwards. It's Good Code.
I have it from good sources that Red Boot is being ported to
Motorola's MBX line, which is both PPC based *and* the (initial)
target of choice for their Embedded DevKit (of which I also have a
copy, it is veeery nice).
In short, you owe it to yourself to check out Red Boot. I don't know
how much overlap there is between Red Boot and PPCBOOT, but I hate to
see you reinvent the wheel if you don't really want to.