In the `HKLM\SYSTEM\CurrentControlSet\Control` registry key,
the `FirmwareBootDevice` value specifies the firmware boot
(i.e. system partition) device in ARC format, obtained from
`LoaderBlock->ArcHalDeviceName`.
For some reason it is exposed only on Windows Vista and later.
This value is similar to the `SystemBootDevice` one, which specifies
instead the OS boot device in ARC format, obtained from
`LoaderBlock->ArcBootDeviceName`.
In addition: check the value returned by `RtlCreateUnicodeStringFromAsciiz()`
and fail if so.