meta-google: Add new system.conf for gbmc-systemd-config

Add systemd.conf options for watchdog to replace watchdog-conf

```
// interval is 60s by default
interval = 10
watchdog-device = /dev/watchdog
// The value is in pages.  They are presently 4KiB each.
// Reset the BMC if it goes below 3MiB
min-memory = 768
```

Tested:
```
$ cat /lib/systemd/system.conf.d/40-system.conf
[Manager]
DefaultMemoryAccounting=yes
RuntimeWatchdogSec=10s
WatchdogDevice=/dev/watchdog
```

Change-Id: I51171746669dfeb1efe6c09952efa3457e33c86b
Signed-off-by: Willy Tu <wltu@google.com>
diff --git a/meta-google/recipes-google/systemd/files/40-system.conf b/meta-google/recipes-google/systemd/files/40-system.conf
new file mode 100644
index 0000000..5f770d0
--- /dev/null
+++ b/meta-google/recipes-google/systemd/files/40-system.conf
@@ -0,0 +1,4 @@
+[Manager]
+DefaultMemoryAccounting=yes
+RuntimeWatchdogSec=10s
+WatchdogDevice=/dev/watchdog