kernel: update k_thread_state_str() API

When threads are in more than one state at a time, k_thread_state_str()
returns a string that lists each of its states delimited by a '+'.
This in turn necessitates a change to the API that includes both a
pointer to the buffer to use for the string and the size of the buffer.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>