otasigcheck: Go back to the key-rewrite check.

Using an intermediate file had the at-the-time unintended but good
side-effect of making execution failures (absent script, no
busybox in recovery, bad selinux setup) pass gracefully. Since this
check is intended as a blacklist, blocking the update when only
sure that breakage will ensue, that was a nice thing to get.

Checking the return code, while cleaner, is making non-key-related
failures like those mentioned above abort execution of the script.
The key itself isn't even checked, yet we're aborting because of it.

So go back to the original behavior: In case we're not sure... pass.

To do:
This can be cleaned up differently, and to the same effect, by returning
an explicit error code from the script and aborting when that error
comes up. Change the script to do that and come back here. :)

Change-Id: Ic9d7c3128047c25f31e2ef416c9df4d21ca0a1a8