scripts/oe-build-perf-report: summary of task resource usage

Utilize buildstats, if available, and show a summary of the resource
usage of bitbake tasks in the html report. The details provided are:
- total number of tasks
- top 5 resource-hungry tasks (cputime)
- top 5 increase in resource usage (cputime)
- top 5 decrease in resource usage (cputime)

[YOCTO #11381]

(From OE-Core rev: ddd9443cb2432af2c15b358bfda708393fa3c417)

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/lib/buildstats.py

@@ -180,6 +180,14 @@ class BSRecipe(object):
 class BuildStats(dict):
     """Class representing buildstats of one build"""
 
+    @property
+    def num_tasks(self):
+        """Get number of tasks"""
+        num = 0
+        for recipe in self.values():
+            num += len(recipe.tasks)
+        return num
+
     @classmethod
     def from_json(cls, bs_json):
         """Create new BuildStats object from JSON object"""