build: clean up otasigcheck

- Only mount and unmount /data if it was originally unmounted
- Don't use comparison hack, just check the result of the script

Change-Id: I4a22485d315cf91e95ce578907c49f5fa3a03222

Conflicts:
	tools/releasetools/edify_generator.py
	tools/releasetools/ota_from_target_files
Revert "build: clean up otasigcheck"

This reverts commit 2e5c8ca6de19d141d320875c2cba9dbd450a8eb2.