[prefix] Delay initrd image copy until memory map is ready

[prefix] Delay initrd image copy until memory map is ready

initrd_init() calls umalloc() to allocate space for the initrd image,
but does so before hide_etherboot() has been called.  It is therefore
possible for the initrd to end up overwriting iPXE itself.

Fix by converting initrd_init() from an init_fn to a startup_fn.

Originally-fixed-by: Till Straumann <strauman@slac.stanford.edu>
Signed-off-by: Michael Brown <mcb30@ipxe.org>