cli: convert to new argparse module

Remove the adt7411 and adt7420 commands. These will be moved into an
example.

Signed-off-by: Michael Walle <michael.walle@kontron.com>
add feature section to README

Signed-off-by: Michael Walle <michael.walle@kontron.com>
test: add missing test cases

Signed-off-by: Michael Walle <michael.walle@kontron.com>