net-lwip: add TFTP_BLOCKSIZE

Add support for setting the TFTP block size. The default value (1468)
is fine for Ethernet and allows a better throughput than the TFTP
default (512), if the server supports the blksize option of course.

I tested this change with qemu_arm64_lwip_defconfig. The throughput is
now 875 KiB/s vs. 313 KiB/s before. That is still a low number, but I
think we can't expect more without implementing the windowsize option.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>