Make the name of the log file produced by launch script configurable

Previously, the launch script would always use a file named
<appname>.log to capture the application's console output. This commit
adds a variable, LOG_FILENAME, for specifying the file name defaulting
to <appname>.log.

Fixes gh-4194

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

@@ -427,6 +427,9 @@ the default behavior in a script or on the command line:
 |`LOG_FOLDER`
 |The name of the folder to put log files in (`/var/log` by default).
 
+|`LOG_FILENAME`
+|The name of the log file in the `LOG_FOLDER` (`<appname>.log` by default).
+
 |`APP_NAME`
 |The name of the app. If the jar is run from a symlink the script guesses the app name,
  but if it is not a symlink, or you want to explicitly set the app name this can be

spring-boot-tools/spring-boot-loader-tools/src/main/resources/org/springframework/boot/loader/tools/launch.script

@@ -61,7 +61,8 @@ if [[ -z "$identity" ]]; then
   fi
 fi
 
-
+# Initialize log file name if not provided by the config file
+[[ -z "$LOG_FILENAME" ]] && LOG_FILENAME="${identity}.log"
 
 
 # ANSI Colors
@@ -91,7 +92,7 @@ if [[ "$identity" == "$init_script" ]] || [[ "$identity" == "$APP_NAME" ]]; then
   PID_FOLDER="$PID_FOLDER/${identity}"
 fi
 pid_file="$PID_FOLDER/${identity}.pid"
-log_file="$LOG_FOLDER/${identity}.log"
+log_file="$LOG_FOLDER/$LOG_FILENAME"
 
 # Determine the user to run as if we are root
 [[ $(id -u) == "0" ]] && run_user=$(ls -ld "$jarfile" | awk '{print $3}')