util/cbfstool: Fix building with clang & -Wshadow

Clang -Wshadow is more rigorous than GCC and picks a shadowing of the
optarg global variable in /usr/include/bits/getopt_core.h .

TESTED: builds with both gcc and clang.

Change-Id: Ifc362c84511abb6a000671f03498e841d7747074
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/70508
Reviewed-by: Kapil Porwal <kapilporwal@google.com>
Reviewed-by: Tarun Tuli <taruntuli@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Subrata Banik <subratabanik@google.com>
diff --git a/util/cbfstool/cbfstool.c b/util/cbfstool/cbfstool.c
index 2dcc9d1..c6e451f 100644
--- a/util/cbfstool/cbfstool.c
+++ b/util/cbfstool/cbfstool.c
@@ -340,9 +340,9 @@
 }
 
 
-static int decode_mmap_arg(char *optarg)
+static int decode_mmap_arg(char *arg)
 {
-	if (optarg == NULL)
+	if (arg == NULL)
 		return 1;
 
 	union {
@@ -354,17 +354,17 @@
 		};
 	} mmap_args;
 	char *suffix = NULL;
-	char *substring = strtok(optarg, ":");
+	char *substring = strtok(arg, ":");
 	for (size_t i = 0; i < ARRAY_SIZE(mmap_args.array); i++) {
 		if (!substring) {
 			ERROR("Invalid mmap arguments '%s'.\n",
-			      optarg);
+			      arg);
 			return 1;
 		}
 		mmap_args.array[i] = strtol(substring, &suffix, 0);
 		if (suffix && *suffix) {
 			ERROR("Invalid mmap arguments '%s'.\n",
-			      optarg);
+			      arg);
 			return 1;
 		}
 		substring = strtok(NULL, ":");
@@ -372,7 +372,7 @@
 
 	if (substring != NULL) {
 		ERROR("Invalid argument, too many substrings '%s'.\n",
-		      optarg);
+		      arg);
 
 		return 1;
 	}