bmc_monitor_app: Support `FirmwarePlusLoader` for Nuvoton SoC

Support `FirmwarePlusLoader` duration for NCPM7XX or newer Nuvoton SoC.
This will mitigate the inaccuracy caused by SoC not supporting Firmware
time and Loader time.

Tested:
\# Check FirmwarePlusLoader time is correct on the BMC with Nuvoton SoC
```
bmc:~# cat /usr/share/boot_time_monitor/bmc_durations.csv.completed
Firmware,0
Loader,0
Kernel,38706
InitRD,0
Userspace,381086
FirmwarePlusLoader,43480
```

Google-Bug-Id: 296530445
Change-Id: Ie340bd5f9d4637f0be1f2915887200b3c3d03e12
Signed-off-by: Michael Shen <gpgpgp@google.com>
6 files changed