When chainloaded from another iPXE, there will be both a virtual
filesystem and a managed network protocol available through which we
could attempt to load autoexec.ipxe.
Try both of these, with the virtual filesystem attempted first so that
an autoexec.ipxe that was explicitly downloaded by the chainloading
iPXE will have the highest priority.
Signed-off-by: Michael Brown <mcb30@ipxe.org>
3df8f9c379