Add a 'fake' go command to the bootm command

For tracing it is useful to run as much of U-Boot as possible so as to get
a complete picture. Quite a bit of work happens in bootm, and we don't want
to have to stop tracing before bootm starts.

Add a way of doing a 'fake' boot of the OS - which does everything up to
the point where U-Boot is about to jump to the OS image. This allows
tracing to record right until the end.

This requires arch support to work.

Signed-off-by: Simon Glass <sjg@chromium.org>