contiki: Add profiling infrastructure

This should be used with gcc's -finstrument-functions
contiki: Add statistical profiler

A periodic interrupt records the value of the program counter.
Add profile-test example to demonstrate the profiling intfrastructure
profiling: Added python script to generate dot graphs
profiling: Use pydot for graph generation, TODO file with open issues
profiling: Change the default number of call sites
avr: Implement clock_fine* functions
profiling: Use clock_fine* to generate more accurate timing information
profiling: Keep track of how long profiling was active
profiling: Print the runtime and tick resolution in the report
tools/profiling: Parse the PROF header and use the time information

Also add times to the function nodes. This is the sum of the incoming
edge times subtracted by the sum of the outgoing edge times.
tools/profiling: Generate initial summary for the statistical profiler
tools/profiling: More TODO notes
tools/profiling: Don't include time for unprofiled functions
tools/profiling: Track number of invocations per function

Also determine whether a function was instrumented or not by counting
number of invocations instead of accumulated time.
tools/profiling: Enable clustering of specific files
tools/profiling: Allow highlighting of certain functions
tools/profiling: Add options for ignoring files/functions
tools/profiling: Update todo
profiling: Keep track of the current call stack