kboot: Call enumerate_memory_arch()

Now that all architectures provide this, enumerate the platform's memory
before we go to interact(). This needs to be done only once, but relies
on our ability to open host: files on some platforms, so it needs to be
done after devinit().

Sponsored by: Netflix
Reviewed by: tsoome, kevans
Differential Revision: https://reviews.freebsd.org/D38251