Previously the end of the heap was the start (lowest address) of the stack.
With the changes in this commit these addresses are now independent,
allowing a board to place the heap and stack in separate locations.
This demonstrates how to use external QSPI flash in XIP (execute in place)
mode. The default configuration has all extmod/ code placed into external
QSPI flash, but other code can easily be put there by modifying the custom
f769_qspi.ld script.