apalis/colibri_t20/t30: increase tftp blocksize

Enable CONFIG_IP_DEFRAG and set CONFIG_TFTP_BLOCKSIZE to 16384.
This increases the tftp download speed considerably.

While at it enable CONFIG_TFTP_TSIZE which limits the progress bar to
fifty '#' independent of the downloaded file size.

Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>