The platform_watchdog_ methods are optional features that the target
platform can choose to implement.
If they are implemented, then BootService.SetWatchdogTimer() would call
them to setup the hardware watchdog.
If they are not implemented (for example presubmit targets), then
BootService.SetWatchdogTimer() would test the WEAK symbol against a NULL
pointer and error out.
To use BootService.SetWatchdogTimer(), platforms can choose to either
implement platform_watchdog_init and platform_watchdog_set_enabled, or
override the entire BootService.SetWatchdogTimer() method.
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