lwIP | lwipopts: MEM_SIZE is not overridable, defaults cannot fit in 512 KiB SRAM. (#19)

[Armaan Chowfin (@ar-in0)]

Issue created by Armaan Chowfin: https://gitlab.rtems.org/rtems/pkg/rtems-lwip/-/work_items/19



## Summary

Two issues related to memory sizing:

a.  `legacy_lwipopts.h` defines a 1 MiB lwIP heap unconditionally (no opt config options either) and the TMS570LC4357 has 512 KiB of SRAM in total. Build fails at link stage.

b. `MEM_SIZE` sets an upper bound for total heap available to lwip during a single run. rx chains at init alloc 10 \* 9600 byte for pbufs under `MEMP_MEM_MALLOC` so we cannot lower the `MEM_SIZE` arbitrarily. 

![image.png](/uploads/efad49e965de93b2508a050ca1eb95f6/image.png){width=900 height=221}

## Steps to reproduce

Run net-ping-shell rtems lwIP app: https://gitlab.rtems.org/ar-in0/rtems-tms570-testing/-/blob/main/net-ping-shell/init.c?ref_type=heads on TI TMS570LC4357 with a debug build of rtems-lwip and rtems.

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View it on GitLab: https://gitlab.rtems.org/rtems/pkg/rtems-lwip/-/work_items/19
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