utils/trace_cmd: reduce regex use

Do not attempt to regex match each line for dropped events/preamble. Use
substring search to detect them first, and only use regex on relevant
lines.

wlauto/utils/trace_cmd.py

@@ -274,11 +274,14 @@ class TraceCmdTrace(object):
                     elif TRACE_MARKER_STOP in line:
                         break
 
+                if 'EVENTS DROPPED' in line:
                     match = DROPPED_EVENTS_REGEX.search(line)
                     if match:
                         yield DroppedEventsEvent(match.group('cpu_id'))
                         continue
 
+                if line.startswith('version') or line.startswith('cpus') or\
+                        line.startswith('CPU:'):
                     matched = False
                     for rx in [HEADER_REGEX, EMPTY_CPU_REGEX]:
                         match = rx.search(line)