sysview: Add support to trace user-defined events
sysview: Add support to trace user-defined events

SysView can now be used to trace/profile any user-defined events.
Event identifier is any number selected by user - SysView client will
match start/stop traces for each identifier and display execution time
as well as run count.

mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core