svc_msg.h: add data for hotplug message
greybus.h: add function prototype for add/remove a module
core.c: create empty functions to keep linking working for hotplug/remove
ap: start validating the message better

We check the type of the message now.

Start to check the size of the payload to match the size of the message
type.  Still more work to do needed here.

Also "hooked up" the hotplug message, but doesn't call anything as the
core doesn't implement that yet...