Use filename like megagrrl_<hwver>_<type>_<version>.mgu if --outfile is unset

Example filenames:
- megagrrl_d_app_v0.98dev-test.mgu
- megagrrl_d_app+updater_v0.98dev-test.mgu
- megagrrl_d_updater_v0.97dev-103-gbbbc2ddf-dirty.mgu

utils/megapacker.pl

@@ -33,8 +33,8 @@ GetOptions(
 (!(defined($app) && defined($updater) && $app eq $updater))
 	or help('--app and --updater must be different files');
 
-(defined($outfile) && ($outfile =~ /.+\.mgu$/) && !(-e $outfile))
-	or help('--outfile must be set, end in .mgu, and not exist');
+(!defined($outfile) || (($outfile =~ /.+\.mgu$/) && !(-e $outfile)))
+	or help('--outfile, if set, must end in .mgu and not exist');
 
 if (defined($hw)) {
 	($hw =~ /^[a-z0-9]{1}$/i)
@@ -71,9 +71,22 @@ else {
 	die('unable to extract version from firmware(s) and --version is not set');
 }
 
+# use the hw type, firmware type(s), and version as the basis for an output filename if one hasn't been explicitly specified
+if (!defined($outfile)) {
+	my @type;
+	(defined($app))
+		and push(@type, 'app');
+	(defined($updater))
+		and push(@type, 'updater');
+
+	$outfile = sprintf('megagrrl_%s_%s_%s.mgu', $hw, join('+', @type), $mgu_version);
+	(-e $outfile)
+		and die('automatically generated output filename ' . $outfile . ' already exists, not overwriting')
+}
+
 print("writing packed firmware to $outfile...\n\n");
 
-my $outfh = FileHandle->new($outfile, 'w')
+my $outfh = FileHandle->new($outfile, O_WRONLY|O_CREAT|O_EXCL)
 	or die("failed to open $outfile: $!");
 
 $outfh->binmode(); # if we're on windows for some reason
@@ -240,7 +253,7 @@ sub help {
 	print STDERR ("--updater=<updater_file.bin> - (optional) the updater portion of the firmware (built with #define FWUPDATE)\n");
 	print STDERR ("--version=some-firmware-version - (optional) the firmware version. if not specified, it will be preferentially extracted from app_file.bin, or updater_file.bin\n");
 	print STDERR ("--hw=<d|p> - (optional) the hardware version, typically one of 'd' for desktop or 'p' for portable. defaults to desktop\n");
-	print STDERR ("--outfile=<output_file.mgu> (required) filename to write the packed (.mgu) firmware to\n");
+	print STDERR ("--outfile=<output_file.mgu> (optional) filename to write the packed (.mgu) firmware to. autogenerated if unset\n");
 	print STDERR ("--help - this message\n");
 
 	exit(defined($error));