net: tftp: Add client support for RFC 7440

Add support for RFC 7440: "TFTP Windowsize Option".

This optional feature allows the client and server
to negotiate a window size of consecutive blocks to send as an
alternative for replacing the single-block lockstep schema.

windowsize can be defined statically during compilation by
setting CONFIG_TFTP_WINDOWSIZE, or defined in runtime by
setting an environment variable: "tftpwindowsize"
If not defined, the windowsize is set to 1, meaning that it
behaves as it was never defined.

Choosing the appropriate windowsize depends on the specific
network topology, underlying NIC.
You should test various windowsize scenarios and see which
best work for you.

Setting a windowsize too big can actually decreases performance.

Signed-off-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Marek Vasut <marex@denx.de>

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