BigBuf and tracing rework: allow much longer traces in in hf commands

- provided a BigBuf_malloc() function to dynamically allocate parts of BigBuf
  e.g. for DMA-Buffers, Frame-Buffers, Emulator-Memory
- the whole rest of BigBuf is now available for traces (instead of a small fixed amount)
- send actual traceLen together with trace data
- changed client side to cope with varying traceLen
- changed small buffers to automatic variables instead of parts of BigBuf