[log] add OT_SHOULD_LOG_AT(aLevel) (#7456)

This commit adds new helper macro `OT_SHOULD_LOG_AT(aLevel)` which
indicates whether logging is enabled at a given log level. This helps
simplify the code and makes sure the the condition used for enabling
logging related functions/methods is consistent across all the core
modules.

With this change, when `OPENTHREAD_CONFIG_LOG_OUTPUT` is set to
`LOG_OUTPUT_NONE` (which practically disables all logging), all the
related code/methods that are used to prepare the log line
(e.g. `Mac::OperationToString()`) are excluded from the build
(become empty functions/methods). This way, under `LOG_OUTPUT = NONE`
instead of preparing the log line and then passing it to an empty
`otPlatLog()` implementation to be dropped, the code is optimized to
not prepare the log line in first place.

src/core/common/notifier.cpp

@@ -216,7 +216,7 @@ exit:
 
 // LCOV_EXCL_START
 
-#if OPENTHREAD_CONFIG_LOG_LEVEL >= OT_LOG_LEVEL_INFO
+#if OT_SHOULD_LOG_AT(OT_LOG_LEVEL_INFO)
 
 void Notifier::LogEvents(Events aEvents) const
 {
@@ -303,7 +303,7 @@ const char *Notifier::EventToString(Event aEvent) const
     return retval;
 }
 
-#else // #if OPENTHREAD_CONFIG_LOG_LEVEL >= OT_LOG_LEVEL_INFO
+#else // #if OT_SHOULD_LOG_AT( OT_LOG_LEVEL_INFO)
 
 void Notifier::LogEvents(Events) const
 {
@@ -314,7 +314,7 @@ const char *Notifier::EventToString(Event) const
     return "";
 }
 
-#endif // #if OPENTHREAD_CONFIG_LOG_LEVEL >= OT_LOG_LEVEL_INFO
+#endif // #if OT_SHOULD_LOG_AT( OT_LOG_LEVEL_INFO)
 
 // LCOV_EXCL_STOP