cmake: sparse: deprecate old sparse support

Deprecate old sparse support as Zephyr now provides a proper
infrastructure for SCA tools. Set ZEPHYR_SCA_VARIANT to sparse if user
is using deprecated way.

This allows to cleanup sparse code in various places and thus have a
cleaner build system.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>