bb-perf: plot histograms base on buildstats data

bb-perf: plot histograms base on buildstats data

Scripts that produces script data to be consumed by gnuplot.
There are two possible plots depending if either the
-S parameter is present or not:

    * without -S: Produces a histogram listing top N recipes/tasks versus
      stats. The first stat defined in the -s parameter is the one taken
      into account for ranking
    * -S: Produces a histogram listing tasks versus stats.  In this case,
      the value of each stat is the sum for that particular stat in all recipes found.
      Stats values  are in descending order defined by the first stat defined on -s

EXAMPLES

1. Top recipes' tasks taking into account utime

    $ buildstats-plot.sh -s utime | gnuplot -p

2. Tasks versus utime:stime

    $ buildstats-plot.sh -s utime:stime -S | gnuplot -p

3. Tasks versus IO write_bytes:IO read_bytes

    $ buildstats-plot.sh -s 'IO write_bytes:IO read_bytes' -S | gnuplot -p

Signed-off-by: Leonardo Sandoval <leonardo.sandoval.gonzalez@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>