With the RTOS, the STACK_SIZE specified here is unrelated to the stack
size available for the main thread (that runs pre_main). Save memory by
reducing the stack size to a more reasonable amount.
On uVisor, HEAP_SIZE is both a minimum available and maximum available
heap size. The heap can't grow beyond the end of the heap into the
neighboring stack. On all uVisor-supported platforms, guarantee at least
0x6000 bytes of heap space. This increases the portability of uVisor
applications as the memory available for legacy heap allocations is
guaranteed. This helps to avoid out of memory errors on platforms that
were previously guaranteeing less memory.
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