Make LOGGING_SYSTEM_BEAN_NAME public

Since the `LoggingSystem` may be retrieved very early during the context
initialization, we should do a lookup by name even if only one instance
of that bean is ultimately expected in the context.

The constant defining the bean name is now public to ease that use case.

Closes gh-4584

spring-boot/src/main/java/org/springframework/boot/logging/LoggingApplicationListener.java

@@ -112,7 +112,7 @@ public class LoggingApplicationListener implements GenericApplicationListener {
 	/**
 	 * The name of the {@link LoggingSystem} bean.
 	 */
-	private static final String LOGGING_SYSTEM_BEAN_NAME = "springBootLoggingSystem";
+	public static final String LOGGING_SYSTEM_BEAN_NAME = "springBootLoggingSystem";
 
 	private static MultiValueMap<LogLevel, String> LOG_LEVEL_LOGGERS;