efi_loader: need either ACPI table or device tree

The EBBR specification prescribes that we should have either an ACPI table
or a device tree but not both. Let us enforce this condition in the
`bootefi` command.

If the bootefi command is called without a device tree parameter use a
previously device tree or fall back to the internal device tree.

The fdt unit test should not be run on boards with an ACPI table.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>