[SL-UP] Lwip config optimizations

[arun-silabs]

Summary

Tune SiWx917 lwIP options for lower static RAM and a correct TCP MSS, while preserving the larger AWS TCP window/send budget.

Changes :

• Raise TCP_MSS to 1460 (Ethernet MTU) for all builds.
• For AWS, set TCP_WND / TCP_SND_BUF explicitly (16× / 8× MSS) so the previous 4×-MSS window/send budget is unchanged without advertising an oversized MSS.
• Cut PBUF_POOL_SIZE (32 to 16) and update PBUF_POOL_BUFSIZE (1280 to 1520) to fit MSS + headers and reduce static pbuf BSS (~41 KB → ~24 KB).

Related issues

MATTER-6533

Testing

Ran extended core tests with thermostat and lock app (3 full suite runs) and verified that no failures related to memory are seen in any of the tests.

[cursor]

PR Summary

Medium Risk
Changes embedded TCP windowing, send buffers, and static pbuf memory, which can affect AWS connectivity and peak RAM under load; scope is limited to SiWx lwIP compile-time options and was exercised in extended Matter app tests.

Overview
SiWx lwIP options (lwipopts-siwx.h) are retuned to use a standard Ethernet TCP_MSS (1460) instead of inflating MSS on AWS builds (previously 4×1152) to grow default window/send sizes.

For SL_MATTER_ENABLE_AWS, TCP_WND and TCP_SND_BUF are now set explicitly (12× and 8× MSS) so AWS can keep a large TCP budget without advertising an oversized MSS. Non-AWS builds set TCP_SND_BUF to 2× MSS and leave TCP_WND at the opt.h default (4× MSS). The duplicate global TCP_SND_BUF define is removed.

Static pbuf RAM is reduced by halving PBUF_POOL_SIZE (32 → 16) and sizing PBUF_POOL_BUFSIZE for MSS plus link/IP headers (1280 → 1520). TCPIP_THREAD_STACKSIZE is unchanged in size (2×1024), and the AWS-only LWIP_DNS 1 line is dropped (DNS now follows the existing LWIP_DNS / LWIP_IPV4 default unless set elsewhere).

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[Rohan Sahay (rosahay-silabs)]

As discussed, if we have the time, can we test this against HEAP3? Rest LGTM

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