ArmVirtPkg/ArmVirtQemu: gate FDT config table install with build option

ArmVirtPkg/ArmVirtQemu: gate FDT config table install with build option

This introduces the .DSC define 'PURE_ACPI_BOOT_ENABLE', defaulting to
FALSE, which controls the value of the feature PCD 'PcdPureAcpiBoot'.

This allows an ArmVirtQemu image to be built that restricts the OS to
booting in ACPI mode.

This feature is only added to ArmVirtQemu, and not to ArmVirtQemuKernel,
the reason being that the latter is mostly intended for development work,
where the burden of adding 'acpi=force' if you need it is much more
tolerable than when trying to boot an installer on a production KVM guest
instance.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>