Introduce @SpringBootConfiguration annotation

Add a new @SpringBootConfiguration annotation that can be used to
indicate the primary application configuration. The new annotation is
primarily indented to allow test automatically code to find the main
configuration class.

See gh-5295

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/SpringBootApplication.java

 /*
- * Copyright 2012-2015 the original author or authors.
+ * Copyright 2012-2016 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -23,6 +23,7 @@ import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
 import java.lang.annotation.Target;
 
+import org.springframework.boot.SpringBootConfiguration;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.ComponentScan;
 import org.springframework.context.annotation.Configuration;
@@ -43,7 +44,7 @@ import org.springframework.core.annotation.AliasFor;
 @Retention(RetentionPolicy.RUNTIME)
 @Documented
 @Inherited
-@Configuration
+@SpringBootConfiguration
 @EnableAutoConfiguration
 @ComponentScan
 public @interface SpringBootApplication {

spring-boot/src/main/java/org/springframework/boot/SpringBootConfiguration.java

+/*
+ * Copyright 2012-2016 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+import org.springframework.context.annotation.Configuration;
+
+/**
+ * Indicates that a class provides Spring Boot application
+ * {@link Configuration @Configuration}. Can be used as an alternative to the Spring's
+ * standard {@code @Configuration} annotation so that configuration can be found
+ * automatically (for example in tests).
+ * <p>
+ * Application should only ever include <em>one</em>
+ * {@code @SpringApplicationConfiguration} and most idiomatic Spring Boot applications
+ * will inherit it from {@code @SpringBootApplication}.
+ *
+ * @author Phillip Webb
+ * @since 1.4.0
+ */
+@Target(ElementType.TYPE)
+@Retention(RetentionPolicy.RUNTIME)
+@Documented
+@Configuration
+public @interface SpringBootConfiguration {
+
+}