Map FATAL level for Log4j

Make sure that Boot's FATAL level is mapped to log4j and log4j2 properly.

Fixes gh-3518

spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -912,6 +912,7 @@ The following items are output:
 * Logger name -- This is usually the source class name (often abbreviated).
 * The log message.
 
+NOTE: Logback does not have a `FATAL` level (it is mapped to `ERROR`)
 
 
 [[boot-features-logging-console-output]]

spring-boot/src/main/java/org/springframework/boot/logging/log4j/Log4JLoggingSystem.java

@@ -49,7 +49,7 @@ public class Log4JLoggingSystem extends Slf4JLoggingSystem {
 		levels.put(LogLevel.INFO, Level.INFO);
 		levels.put(LogLevel.WARN, Level.WARN);
 		levels.put(LogLevel.ERROR, Level.ERROR);
-		levels.put(LogLevel.FATAL, Level.ERROR);
+		levels.put(LogLevel.FATAL, Level.FATAL);
 		levels.put(LogLevel.OFF, Level.OFF);
 		LEVELS = Collections.unmodifiableMap(levels);
 	}

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

@@ -64,7 +64,7 @@ public class Log4J2LoggingSystem extends Slf4JLoggingSystem {
 		levels.put(LogLevel.INFO, Level.INFO);
 		levels.put(LogLevel.WARN, Level.WARN);
 		levels.put(LogLevel.ERROR, Level.ERROR);
-		levels.put(LogLevel.FATAL, Level.ERROR);
+		levels.put(LogLevel.FATAL, Level.FATAL);
 		levels.put(LogLevel.OFF, Level.OFF);
 		LEVELS = Collections.unmodifiableMap(levels);
 	}