If a load-only FIT has already provided a devicetree, PXE boot may need
to restart the bootm sequence, rather than starting an entirely new one,
so that the devicetree is preserved and used for booting.
Add support for this by adding a new field in the context and updating
extlinux_boot() to receive the value as a new parameter.
Signed-off-by: Simon Glass <sjg@chromium.org>