perfboot.py: Add --end-tag option

The current behavior, which is to use tags[-1] for end_tag, is good
enough in most cases, but sometime it makes more sense to use a
different one as end_tag.

For example, when optimizing APK scanning code in package manager,
using 'boot_progress_pms_scan_end' as end_tag would give us quicker
benchmark iterations as well as more relevant numbers possibly with
a smaller standard deviation.

Change-Id: Ic6371c936b36d1d87e9742502fd8958682a3b7d5