Add GBL_EFI_FASTBOOT_PROTOCOL.should_stop_in_fastboot

Update the `GBL_EFI_FASTBOOT_PROTOCOL` to include a
`should_stop_in_fastboot` method.

This method checks for a device signal during boot, usually a specific
button press combination, to determine whether the device should stop
in fastboot mode.

Tests: new unit tests pass

Change-Id: Ic457e8bfc8892b2c56d561b303ccc09dddba74fb