console: Add a console buffer

It is useful to be able to record console output and provide console input
via a buffer. This provides sandbox with the ability to run a command and
check its output. If the console is set to silent then no visible output
is generated.

This also provides a means to fix the problem where tests produce unwanted
output, such as errors or warnings. This can be confusing. We can instead
set the console to silent and record this output. It can be checked later
in the test if required.

It is possible that this may prove useful for non-test situations. For
example the console output may be suppressed for normal operations, but
recorded and stored for access by the OS. That feature is not implemented
at present.

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