refactor(qemu): guard handoff logic w/ build flag

Prepare for environments where the Firmware Handoff (LibTL)
submodule may not be available.  Wrap all Transfer List dependent logic
in `#if TRANSFER_LIST` guards, ensuring QEMU can build and run without
the submodule.

This is useful for builds not integrating the firmware handoff
mechanism.

Change-Id: I0be3773bf300b02cd3beccf738a021925e3c53c6
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
2 files changed