convert configure() to property

Instead of having one configure() method, we now use enable_i2c and
enable_spi properties, which can be set to True.

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

Remove unneeded import statements and one converstion which is not needed
anymore, thanks to the new argument parser.

Signed-off-by: Michael Walle <michael.walle@kontron.com>
cli: accept an args argument on main()

This will ease the testing.

Signed-off-by: Michael Walle <michael.walle@kontron.com>
cli: make use of return codes

Return 1 in case of an error.

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