dbus-python: add host package

dbus-python: add host package

This can be useful for Python scripts running on a host machine that
need to connect to a bus on a remote device, e.g. test scripts.

The target dbus-python depends on dbus instead of selecting it because
dbus has dependencies that check that it is supported. Using 'select'
would override those dependencies. For host-dbus, there are no such
checks -- it's assumed the host has the features neeeded to run dbus, so
it is okay to just depend on host-dbus.

The pyexpat dependency is not needed, as it is forced on for
host-python.

Signed-off-by: Brent Generous <bgenerous@impinj.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>