basic simulator: initial release

This commit introduces a 'basic simulator'.

This basic simulator is made of:
- the standard eNB code using a special driver that mimics the USRP driver
- the standard UE code using a special driver that mimics the USRP driver
- no channel simulation
- some special code to deal with faster-than-realtime behaviour of this
  basic simulator

It connects one UE to one eNB. It requires an EPC, populated with the
correct configuration for the UE.

This is the initial release and may contain bugs (most probably race
conditions due to the faster-than-realtime behaviour).

To use it, see the documentation at:
    targets/ARCH/tcp_bridge/README.tcp_bridge_oai.

It has been tested with 25, 50 and 100 RBs, FDD mode.
(No check at all has been done to know if it could work in TDD mode.)

common/utils/T/T.h

@@ -110,6 +110,16 @@ typedef struct {
 extern volatile int *T_freelist_head;
 extern T_cache_t *T_cache;
 
+/* When running the basic simulator, we may fill the T cache too fast.
+ * Let's not crash if it's full, just wait.
+ */
+#if BASIC_SIMULATOR
+#  define T_BASIC_SIMULATOR_WAIT \
+     while (T_cache[T_LOCAL_slot].busy) usleep(100)
+#else
+#  define T_BASIC_SIMULATOR_WAIT /* */
+#endif
+
 /* used at header of Tn, allocates buffer */
 #define T_LOCAL_DATA \
   char *T_LOCAL_buf; \
@@ -118,6 +128,7 @@ extern T_cache_t *T_cache;
   T_LOCAL_slot = __sync_fetch_and_add(T_freelist_head, 1) \
                  & (T_CACHE_SIZE - 1); \
   (void)__sync_fetch_and_and(T_freelist_head, T_CACHE_SIZE - 1); \
+  T_BASIC_SIMULATOR_WAIT; \
   if (T_cache[T_LOCAL_slot].busy) { \
     printf("%s:%d:%s: T cache is full - consider increasing its size\n", \
            __FILE__, __LINE__, __FUNCTION__); \