greybus: skeleton for future uevents.

Implement a skeleton for the uevent framework, to be filled in later
when we figure out what type of module "matching" we want to do for
things (connections, interfaces, modules, etc.)

Based on a patch from Viresh Kumar <viresh.kumar@linaro.org>

Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>