releasetools: Look for recovery.fstab at both locations.

The change in [1] moved the recovery etc files from /etc to /system/etc.
However, we may use the latest OTA tools to build incremental OTAs for
old target_files zips. This CL adds a workaround to look at both of the
old and new locations.

[1] commit 696bb336

Bug: 113073663
Test: Build a previously failing incremental OTA for taimen.
Change-Id: Ie07aa1713e616d523838b1260a992b20f5a11612