"view recovery logs" will show /tmp/recovery.log

For A/B devices, "view recovery logs" doesn't work due to the lack
of cache partition. To help debugging, we'll show /tmp/recovery.log
instead if /cache is not found.

Change-Id: Idb77c3a4c30388148a210b38d732a7b27e757bba
Test: Tested on an A/B device and /tmp/recovery.log showed up.
Bug: 30905700
(cherry picked from commit a54f75ed)