allow EventLog tag numbers to be auto-assigned

With this change, you can specify "?" in place of a tag number in a
.logtags file and the build system will assign numbers to these tags.
(The numbers used shouldn't matter since we translate them back to tag
names whenever the logs are read back.)

This is pretty straightforward to do:

- make merge-event-log-tags.py assign numbers to any tags that specify
  "?"

- make the generated java files depend on the merged output

- make java-event-log-tags.py read both the original .logtags and the
  merged output, and fill in tag numbers for any "?" using the merged
  version.

Change-Id: Icc6ccd705db461d570fc929922a830aa6deaca48