Convert license tags to SPDX identifiers

This commit also converts our own copyright headers to directly use
SPDX, but leaves all other copyright header intact, only adding the
SPDX ident. As far as possible this commit also merges multiple
Data61 copyright statements/headers into one for consistency.

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)

SMP/x86: convention - put all ipi.* files under smp/ directory

This follows the convention of other higher-level ipi.* smp.* files
put under smp/. Also upcoming ARM files.