* AR_LEN can be used directly for countdown var

* tick-tock buffer approach: one buffer is filled via AXI4 read
  bursts, the other is made available for dequeuing
* can provide continuous data at full speed, given that the bursts
  finish fast enough
* also added some unit tests for fixed blocks of memory
* missing: support for wrap-around (currently via reset)

* memory size can now be controlled independently from addrWidth
  to allow large address spaces

* adapter to write data from Decoupled-Fifo as AXI4 master
* base address supplied as input
* all widths configurable
* FifoAxiAdapterTest1 uses DecoupledDataSource to test with
  constant data set
* can also be used to verify against AXI BFMs