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/cmdmain.c

@@ -188,7 +188,6 @@ void UsbCommandReceived(UsbCommand *UC)
 		} break;
 
 		case CMD_DOWNLOADED_RAW_ADC_SAMPLES_125K: {
-			sample_buf_len += UC->arg[1];
 			memcpy(sample_buf+(UC->arg[0]),UC->d.asBytes,UC->arg[1]);
 		} break;