TFTP block size has a big impact on the transmit performance, this patch is to add new argument [-s <block size>] for shell "tftp" command to configure the block size for file download. Contributed-under: TianoCore Contribution Agreement 1.0 Signed-off-by: Fu Siyuan <siyuan.fu@intel.com> Reviewed-by: Ye Ting <ting.ye@intel.com> Reviewed-by: Qiu Shumin <shumin.qiu@intel.com> Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>