[RFC] redboot image inplace update

[Jani Monoses <jani at iv dot ro>]

For some RAM constrained targets it would be nice not have to load a
redboot ram image (or running romram) and the new redboot rom image
because they might not both fit.

Even if they would it's nice to have such a feature for when you don't
want to store a ram image permanently in flash or download it via
serial. And in fact it might be useful for all targets which normally
run redboot from flash.

It's done by adding a do_update redboot command along with a helper
function which runs out of RAM and erases the flash under redboot and
rewrites it, than hangs (optionally reset?).

This is a sketch of what the final patch would look like but I thought
I'd ask if people think it's worth doing differently or worth
including at all in CVS or point out things I might have not
considered. I know it uses the flash device directly without going
through io/flash but those functions live in ROM.

thanks
Jani

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