extmod/vfs_fat: Set default volume label on mkfs if it's defined.

Using mkfs doesn't set a volume label for FAT filesystems.  This commit
will set the volume label if `MICROPY_HW_FLASH_FS_LABEL` is defined.

extmod/vfs_fat.c

@@ -114,6 +114,11 @@ static mp_obj_t fat_vfs_mkfs(mp_obj_t bdev_in) {
         mp_raise_OSError(fresult_to_errno_table[res]);
     }
 
+    // set the filesystem label if it's configured
+    #ifdef MICROPY_HW_FLASH_FS_LABEL
+    f_setlabel(&vfs->fatfs, MICROPY_HW_FLASH_FS_LABEL);
+    #endif
+
     return mp_const_none;
 }
 static MP_DEFINE_CONST_FUN_OBJ_1(fat_vfs_mkfs_fun_obj, fat_vfs_mkfs);