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

client/data.c

@@ -16,11 +16,9 @@
 #include "cmdmain.h"
 
 uint8_t* sample_buf;
-size_t sample_buf_len;
 
 void GetFromBigBuf(uint8_t *dest, int bytes, int start_index)
 {
-  sample_buf_len = 0;
   sample_buf = dest;
   UsbCommand c = {CMD_DOWNLOAD_RAW_ADC_SAMPLES_125K, {start_index, bytes, 0}};
   SendCommand(&c);