Access the device tree blob via copyin/copyout.

Access the device tree blob via copyin/copyout.

The code previously assumed that copyin/copyout did no
address translation and that the device tree blob could
be manipulated in-place (with only a few adjustments for
the ELF loader offset).  This isn't possible on all platforms,
so the revised code uses copyout() to copy the device tree
blob into a heap-allocated buffer and then updates the
device tree with copyout().  This isn't ideal, since it
bloats the loader memory usage, but seems the only feasible
approach (short of rewriting all of the fdt manipulation
routines).