bmc_monitor_app: Add `UserspaceEnd` checkpoint in BMC boot flow

Adding `UserspaceEnd` for more detailed boot flow in BMC.

Tested:
```
platform11-bmc:~# busctl --verbose call com.google.gbmc.boot_time_monitor \
/xyz/openbmc_project/time/boot/bmc \
xyz.openbmc_project.Time.Boot.Checkpoint \
GetCheckpointList
MESSAGE "a(sxx)" {
        ARRAY "(sxx)" {
                STRUCT "sxx" {
                        STRING "UserspaceEnd";
                        INT64 1714449201742;
                        INT64 497340;
                };
        };
};
```

Google-Bug-Id: 324155294
Change-Id: I9816d97ad5edaa6eecfb43f1a11b631dae1e330f
Signed-off-by: Michael Shen <gpgpgp@google.com>
1 file changed