backuptool: Add an option to ensure running.

When updating via ROMManager, the presence of /tmp/.installscript
will keep backuptool from running. For people who don't like this
happen can `touch /system/etc/force_backuptool` to ensure the
running of backuptool.

Change-Id: I95b78ae569405b50d2cf95ddd05f0d554b32daad
1 file changed