sandbox: Add a flag to set the default log level

It is useful to be able to set the default log level from the command line
when running sandbox. Add a new -L command-line flag for this. The log
level is set using the enum log_level_t in log.h. At present a number must
be specified, e.g. -L7 for debug.

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