tracing: Add user-definable tracing interface

Create a new tracing option TRACING_USER that allows
the user to define certain user_sys_trace_... functions
to perform whatever work desired for tracing when
tasks are swiched in/out, during isr enter/exit, and when
cpu is idle.

This infrastructure can be useful for plugging into
locally defined tracing tools or any user-specific
debugging environment.

Signed-off-by: Nicholas Lowell <nlowell@lexmark.com>