kernel: Swap RUNTIME_STATS implementation

Clean up RUNTIME_STATS to separate the API from the individual data
backends.  Use the SCHED_THREAD_USAGE tracking instead of the original
for execution_cycles.  Move the kconfig for that into the runtime
stats menu, since it's part of the family now.

Also remove a lot of needless #if's around the declarations.  Unused
structs and uncalled functions don't need to be explicitly hidden.  An
attempt to access a non-existent field (e.g. "execution_cycles" if
that isn't configured) provides all the build time validation we need.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>
5 files changed