Bluetooth: btp: Add IUT ready event

Before sending any command, the tester shall wait for this event
indicating that IUT is ready to accept first BTP command.
This is needed, because some devices need more time to initialize
their stacks, so that the first BTP command can be lost.

Change-Id: I92d1c6ea8be905734037eb29f7f270a70ab1583e
Signed-off-by: Mariusz Skamra <mariusz.skamra@gmail.com>