Make event queue use RTOS tick count

Event queue was using its own Timer or LowPowerTimer objects to derive
millisecond tick counts. This is unnecessary in RTOS builds, where the
RTOS is maintaining a tick count.

It also makes more sense to use the actual RTOS tick count, as the
values are being used to compute tick timeouts for RTOS calls. Computing
these RTOS tick delays with a separate timer could conceivably lead to
rounding errors.

Fixes: #5378

events/equeue/equeue_mbed.cpp

@@ -23,6 +23,15 @@
 #include <stdbool.h>
 #include "mbed.h"
 
+// Ticker operations
+#if MBED_CONF_RTOS_PRESENT
+
+unsigned equeue_tick() {
+    return osKernelGetTickCount();
+}
+
+#else
+
 #if MBED_CONF_EVENTS_USE_LOWPOWER_TIMER_TICKER
 #define ALIAS_TIMER      LowPowerTimer
 #define ALIAS_TICKER     LowPowerTicker
@@ -33,7 +42,6 @@
 #define ALIAS_TIMEOUT    Timeout
 #endif
 
-// Ticker operations
 static bool equeue_tick_inited = false;
 static volatile unsigned equeue_minutes = 0;
 static unsigned equeue_timer[
@@ -77,6 +85,7 @@ unsigned equeue_tick() {
     return minutes + ms;
 }
 
+#endif
 
 // Mutex operations
 int equeue_mutex_create(equeue_mutex_t *m) { return 0; }

events/mbed_lib.json

@@ -23,7 +23,7 @@
             "value": 256
         },
         "use-lowpower-timer-ticker": {
-            "help": "Enable use of low power timer and ticker classes. May reduce the accuracy of the event queue.",
+            "help": "Enable use of low power timer and ticker classes in non-RTOS builds. May reduce the accuracy of the event queue. In RTOS builds, the RTOS tick count is used, and this configuration option has no effect.",
             "value": 0
         }
     }