coredump-config: Move to tmpfs in ExecStopPost

Currently, the coredump-config service is configured to only move files
to tmpfs if the service has already started up successfully. If you try
to stop the service before it's done starting up, the ExecStop command
won't run, which leaves a dangling reference to the eMMC LUKS device,
and as a result, we can't deactivate the LUKS device.

This commit changes the ExecStop entry to an ExecStopPost entry, so that
command will run regardless of whether the startup was successful, and
regardless of whether the startup already finished.

Tested:
Repeatedly formatted and immediately locked the eMMC 500 times.

presubmit:
https://fusion2.corp.google.com/fcdced94-95f1-3ff2-a06b-e1b3a1586d8e
https://fusion2.corp.google.com/e1215315-822e-317f-b6ef-2a24f6b62d8a
https://fusion2.corp.google.com/ae767654-243f-317a-9904-10c872880a7e
https://fusion2.corp.google.com/0533ae54-a36d-3cd3-adb8-6ecc7d3ea6fc

Google-Bug-Id: 385091808
Google-Bug-Id: 363092253
Google-Bug-Id: 384881453
Change-Id: Ied662705160dff040af8d101931a632af817c435
Signed-off-by: John Wedig <johnwedig@google.com>
diff --git a/recipes-google/coredump-config/coredump-config/coredump-config.service b/recipes-google/coredump-config/coredump-config/coredump-config.service
index abd0f6b..358a4d4 100644
--- a/recipes-google/coredump-config/coredump-config/coredump-config.service
+++ b/recipes-google/coredump-config/coredump-config/coredump-config.service
@@ -8,7 +8,7 @@
 Type=oneshot
 RemainAfterExit=yes
 ExecStart=/usr/bin/coredump-config.sh emmc
-ExecStop=/usr/bin/coredump-config.sh tmpfs
+ExecStopPost=/usr/bin/coredump-config.sh tmpfs
 ExecStartPost=systemctl restart obmc-dump-monitor.service
 
 [Install]