mkimage: fix display of image types list

Since commit 5b9d44df23 (mkimage: Display a better list of available image
types) mkimage usage text suggest to "use -T to see a list of available image
types". Unfortunately, commit 02221f29deb8 (mkimage: Convert to use getopt())
broke that feature, because getopt() fails when -T has no option argument.

Add a pseudo image type name 'list' that lists all image types. Update the
usage text accordingly.

Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>

tools/mkimage.c

@@ -113,7 +113,7 @@ static void usage(const char *msg)
 #endif
 	fprintf(stderr, "       %s -V ==> print version information and exit\n",
 		params.cmdname);
-	fprintf(stderr, "Use -T to see a list of available image types\n");
+	fprintf(stderr, "Use '-T list' to see a list of available image types\n");
 
 	exit(EXIT_FAILURE);
 }
@@ -260,6 +260,10 @@ static void process_args(int argc, char **argv)
 			params.skipcpy = 1;
 			break;
 		case 'T':
+			if (strcmp(optarg, "list") == 0) {
+				show_valid_options(IH_TYPE);
+				exit(EXIT_SUCCESS);
+			}
 			type = genimg_get_type_id(optarg);
 			if (type < 0) {
 				show_valid_options(IH_TYPE);