tools/mkenvimage.c: Fix a merge issue

With bfcc40bb09b05c90cc3b1496abb270eb8aa72134 'optopt' was reverted.

Signed-off-by: Horst Kronstorfer <hkronsto@frequentis.com>
diff --git a/tools/mkenvimage.c b/tools/mkenvimage.c
index c5ed373..8ee2bd0 100644
--- a/tools/mkenvimage.c
+++ b/tools/mkenvimage.c
@@ -122,11 +122,11 @@
 			return EXIT_SUCCESS;
 		case ':':
 			fprintf(stderr, "Missing argument for option -%c\n",
-				option);
+				optopt);
 			usage(argv[0]);
 			return EXIT_FAILURE;
 		default:
-			fprintf(stderr, "Wrong option -%c\n", option);
+			fprintf(stderr, "Wrong option -%c\n", optopt);
 			usage(prg);
 			return EXIT_FAILURE;
 		}