sandbox: Allow puts() output before global_data is set up

We support putc() in this case but not puts(), but this is more useful
since it is what printf() uses.

This particularly affects debugging early in SPL, where currently printf()
statements result in no output. Fix this by adding a special case into
puts() for sandbox, just like putc().

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>