net: convert TFTP_TSIZE to proper Kconfig option

TFTP transfer size can be used to re-size the TFTP progress bar on
single line based on the server reported file size. The support for
this has been around from 2019, but it was never converted to proper
Kconfig.

While adding this new Kconfig, enable it by default for OMAP2+ and K3
devices also.

Signed-off-by: Tero Kristo <kristo@kernel.org>