Defer SpringBootServletInitializer getLog

Change `SpringBootServletInitializer` so that the logger is created
in `onStartup` rather than on class creation. The allows logging to
be initialized in an different WebApplicationInitializer.

Fixes gh-3704

spring-boot/src/main/java/org/springframework/boot/context/web/SpringBootServletInitializer.java

@@ -62,7 +62,7 @@ import org.springframework.web.context.WebApplicationContext;
  */
 public abstract class SpringBootServletInitializer implements WebApplicationInitializer {
 
-	protected final Log logger = LogFactory.getLog(getClass());
+	protected Log logger; // Don't initialize early
 
 	private boolean registerErrorPageFilter = true;
 
@@ -78,6 +78,9 @@ public abstract class SpringBootServletInitializer implements WebApplicationInit
 
 	@Override
 	public void onStartup(ServletContext servletContext) throws ServletException {
+		// Logger initialization is deferred in case a ordered
+		// LogServletContextInitializer is being used
+		this.logger = LogFactory.getLog(getClass());
 		WebApplicationContext rootAppContext = createRootApplicationContext(
 				servletContext);
 		if (rootAppContext != null) {