[efi] Create a copy of the system flattened device tree, if present

[efi] Create a copy of the system flattened device tree, if present

EFI configuration tables may be freed at any time, and there is no way
to be notified when the table becomes invalidated.  Create a copy of
the system flattened device tree (if present), so that we do not risk
being left with an invalid pointer.

Signed-off-by: Michael Brown <mcb30@ipxe.org>