SOFTSIM_STACK_SIZE is excessive

[JordanYates]

As the comment says, 10000 is an excessive value for SOFTSIM_STACK_SIZE, over 5% of the total RAM available to an application (ignoring the heap used for message passing).
Regardless of whether this value can be optimised at this point, the value should be swapped to a Kconfig symbol so that the value can be updated as needed by users.