kconfig: use true and false for bool variable

fixed the following coccicheck:
./scripts/kconfig/confdata.c:36:9-10: WARNING: return of 0/1 in function
'is_dir' with return type bool

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/kconfig/confdata.c

@@ -33,7 +33,7 @@ static bool is_dir(const char *path)
 	struct stat st;
 
 	if (stat(path, &st))
-		return 0;
+		return false;
 
 	return S_ISDIR(st.st_mode);
 }