ArmVirtPkg/FdtClientDxe: make DT table installation !ACPI dependent

Instead of having a build time switch to prevent the FDT configuration
table from being installed, make this behavior dependent on whether we
are passing ACPI tables to the OS. This is done by looking for the
ACPI 2.0 configuration table, and only installing the FDT one if the
ACPI one cannot be found.

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