[PATCH build] Implement auto-clone.

[Trevor Woerner]

I already have a sample patch to build.sh where you specify a list of
modules to build on the cmdline and it builds them in the correct
order. If my auto-clone and auto-config patches were in it would also
support those too. Changing it so the list is supplied through a file
would be simple enough. Adding a pretty front-end to generate the file
could be added as well (aka Kconfig).

This was done within the existing functionality of build.sh, so the
list doesn't need to be removed from build.sh or stored externally to
it in any way. Just a small patch to the existing build.sh is all that
is required.