cmd: log: Add commands to manipulate filters

This adds several commands to add, list, and remove log filters. Due to the
complexity of adding a filter, `log filter-list` uses options instead of
positional arguments.

These commands have been added as subcommands to log by using a dash to
join the subcommand and subsubcommand. This is stylistic, and they could be
converted to proper subsubcommands if it is wished.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>