greybus: loopback: add bitmask of connections to include in test

Feature add which enables the ability to select a bit-mask of connections
to run when executing a loopback test set. This is a feature add to
facilitate testing on the firmware side minus the necessity to recompile
firmware to support unicast (v) multicast (v) bitmask.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>