systemd: Use overlay for specifying watchdog parameters

There's no reason to sed stuff. This way also makes it easier to support
running with empty /etc.

Change-Id: I16c0dba53682df574ee119951f47c7c6d4071d61
diff --git a/board/czechlight/common/overlay/usr/lib/systemd/system.conf.d/watchdog.conf b/board/czechlight/common/overlay/usr/lib/systemd/system.conf.d/watchdog.conf
new file mode 100644
index 0000000..3c8a109
--- /dev/null
+++ b/board/czechlight/common/overlay/usr/lib/systemd/system.conf.d/watchdog.conf
@@ -0,0 +1,3 @@
+[Manager]
+RuntimeWatchdogSec=60
+ShutdownWatchdogSec=60