tower: add channelSourceCallback api and implementation
ivory-tower: add signalInit api

* refactors taskInit into nodeInit
* taskst_taskinit now nodest_nodeinit
tower-freertos: implement changes for generalized node init

* note that all nodes initialize before starting loops
* signals init from task land, so it is not safe to use
  communication primitives from a signalInit. Maybe find a
  hack to fix this?
ivory-tower-aadl: fixes for generalized node inits