Automatically detect log4j2.properties

See gh-16262

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/log4j2/Log4J2LoggingSystem.java

@@ -112,6 +112,7 @@ public class Log4J2LoggingSystem extends Slf4JLoggingSystem {
 
 	private String[] getCurrentlySupportedConfigLocations() {
 		List<String> supportedConfigLocations = new ArrayList<>();
+		supportedConfigLocations.add("log4j2.properties");
 		if (isClassAvailable("com.fasterxml.jackson.dataformat.yaml.YAMLParser")) {
 			Collections.addAll(supportedConfigLocations, "log4j2.yaml", "log4j2.yml");
 		}

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/logging/log4j2/Log4J2LoggingSystemTests.java

@@ -211,7 +211,7 @@ public class Log4J2LoggingSystemTests extends AbstractLoggingSystemTests {
 	@Test
 	public void configLocationsWithNoExtraDependencies() {
 		assertThat(this.loggingSystem.getStandardConfigLocations())
-				.contains("log4j2.xml");
+				.contains("log4j2.properties", "log4j2.xml");
 	}
 
 	@Test
@@ -241,7 +241,8 @@ public class Log4J2LoggingSystemTests extends AbstractLoggingSystemTests {
 	@Test
 	public void springConfigLocations() {
 		String[] locations = getSpringConfigLocations(this.loggingSystem);
-		assertThat(locations).isEqualTo(new String[] { "log4j2-spring.xml" });
+		assertThat(locations).isEqualTo(
+				new String[] { "log4j2-spring.properties", "log4j2-spring.xml" });
 	}
 
 	@Test