usec timer: add support to reset time

* initUsecTimer -> usecTimerInit for consistency
* new (public) function to reset the timer to 0
* new parameter "usec.reset" to trigger a reset

The latter an easy way to synchronize the clocks of multiple UAVs, by
sending a broadcast parameter change.

src/hal/interface/usec_time.h

@@ -7,7 +7,7 @@
  *
  * Crazyflie control firmware
  *
- * Copyright (C) 2011-2021 Bitcraze AB
+ * Copyright (C) 2011-2023 Bitcraze AB
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -30,7 +30,12 @@
 /**
  * Initialize microsecond-resolution timer (TIM1).
  */
-void initUsecTimer(void);
+void usecTimerInit(void);
+
+/**
+ * Reset the microsecond-resolution timer to 0.
+ */
+void usecTimerReset(void);
 
 /**
  * Get microsecond-resolution timestamp.

src/hal/src/usec_time.c

@@ -7,7 +7,7 @@
  *
  * Crazyflie control firmware
  *
- * Copyright (C) 2011-2021 Bitcraze AB
+ * Copyright (C) 2011-2023 Bitcraze AB
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -27,14 +27,16 @@
 #include <stdbool.h>
 #include "usec_time.h"
 #include "cfassert.h"
+#include "param.h"
 
 #include "nvicconf.h"
 #include "stm32fxxx.h"
 
 static bool isInit = false;
+static uint8_t reset = 0;
 static uint32_t usecTimerHighCount;
 
-void initUsecTimer(void)
+void usecTimerInit(void)
 {
   if (isInit) {
     return;
@@ -71,6 +73,16 @@ void initUsecTimer(void)
   isInit = true;
 }
 
+void usecTimerReset(void)
+{
+  IF_DEBUG_ASSERT(isInit);
+
+  const uint32_t zero = 0;
+  __atomic_store(&usecTimerHighCount, &zero, __ATOMIC_SEQ_CST);
+
+  TIM7->CNT = 0;
+}
+
 uint64_t usecTimestamp(void)
 {
   IF_DEBUG_ASSERT(isInit);
@@ -96,3 +108,25 @@ void __attribute__((used)) TIM7_IRQHandler(void)
 
   __sync_fetch_and_add(&usecTimerHighCount, 1);
 }
+
+/**
+ * Parameters for the usec timer
+ * */
+static void resetParamCallback(void)
+{
+  if (reset) {
+    usecTimerReset();
+    reset = 0;
+  }
+}
+
+PARAM_GROUP_START(usec)
+
+/**
+ * @brief Reset the time to zero.
+ * 
+ * Useful for time synchronization between UAVs, if reset is send as a broadcast.
+ */
+PARAM_ADD_WITH_CALLBACK(PARAM_UINT8, reset, &reset, &resetParamCallback)
+
+PARAM_GROUP_STOP(usec)