boot: load device tree even if no original config exists

boot: load device tree even if no original config exists

Firmware may not have loaded a devicetree, for example if the device
shipped with windows and exclusively supports ACPI.
We should always load the specified devicetree regardless of firmware
state to enable booting on platforms where Linux only supports DT.

Note: in _cleanup, the orig. config is NULL in this case, and passing
NULL to InstallConfigurationTable is permitted by the EFI spec.
See: https://uefi.org/specs/UEFI/2.10/07_Services_Boot_Services.html

Fixes #24059

Co-authored-by: Daniel Thompson <daniel.thompson@linaro.org>