SMP: Introduce ENABLE_SMP_SUPPORT

- Make it more readable and less confusing compared to the 'CONFIG_MAX_NUM_NODES > 1' check

Fix licenses (the rest)

s/D61/DATA61/ in license headers for consistency

Unify kernel stack definition/declaration and share it between architecures/modes

Rather than defining a kernel stack in random places for each
architecure for each mode (32/64) and for single/SMP modes, make the
stack definition shareable between all of the above. This is also useful
for the future ARM SMP work.