[examples] Introducing pseudo resets (#2621)

Pseudo resets allow for `otPlatReset()` to reset the OpenThread state
without resetting the chip. This is important if the SoC is presenting
itself as a USB device.

The build option `OPENTHREAD_PLATFORM_USE_PSEUDO_RESET` can be used to
force this feature to be enabled or disabled by setting it to `1` or
`0` accordingly. Otherwise it will be set to a platform-specified
default value.

examples/apps/cli/main.c

@@ -63,6 +63,8 @@ int main(int argc, char *argv[])
     uint8_t *otInstanceBuffer       = NULL;
 #endif
 
+pseudo_reset:
+
     PlatformInit(argc, argv);
 
 #if OPENTHREAD_ENABLE_MULTIPLE_INSTANCES
@@ -86,17 +88,19 @@ int main(int argc, char *argv[])
     otDiagInit(sInstance);
 #endif
 
-    while (1)
+    while (!PlatformPseudoResetWasRequested())
     {
         otTaskletsProcess(sInstance);
         PlatformProcessDrivers(sInstance);
     }
 
-        // otInstanceFinalize(sInstance);
+    otInstanceFinalize(sInstance);
 #if OPENTHREAD_ENABLE_MULTIPLE_INSTANCES
-        // free(otInstanceBuffer);
+    free(otInstanceBuffer);
 #endif
 
+    goto pseudo_reset;
+
     return 0;
 }