[RFC] Support multiple environments in barebox

[Juergen Beisert]

This patch series adds handling routines for multiple environments. This
can be usefull if an environment partition gets destroyed by accident. In
this case barebox falls back to a second (or more) environment partition.

The board file just have to register env0, env1 and so on. When running the
saveenv command, barebox will store the new content to all environment
partitions it finds.

Comments are welcome, as this is a cleaned patch stack from one of March
last year. And untested, due to the lack of hardware.

jbe