Fix the bug I introduced in r247045.
After digging through more carefully, it looks like there's
no real need to have the DTB in the module directory.
So we can simplify a lot: Just copy DTB into local heap
for "fdt addr" and U-Boot integration, drop all the extra
COPYIN() calls.

I've left one final COPYIN() to update the in-kernel DTB
for consistency with how this code used to work, but I'm
no longer convinced it's appropriate here.

I've also remove the mem_load_raw() utility that I added
to boot/common/module.c with r247045 since it's no longer
necessary.