crash-reporter: fix pstore detection

The current Enable logic looks for an existing crash rather than the
crash dir itself, so when we boot up clean we get a warning that we
do not support kernel crashes.

BUG=chromium:417350
TEST=`FEATURES=test emerge-link crash-reporter` passes
TEST=`cbuildbot amd64-generic-full` passes
TEST=booted VM and checked syslogs

Change-Id: I076a37889ea10ed30ae522eba007619b001ec787
Reviewed-on: https://chromium-review.googlesource.com/219751


Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Luigi Semenzato <semenzato@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>