upgrade stable/12 -> stable/13 zfs + boot partition Mediasize 64K

[Matthew D. Fuller]

On Thu, Feb 11, 2021 at 04:43:57PM -0800 I heard the voice of
Freddie Cash, and lo! it spake thus:
>
> Sorry, meant 256 KB or 512 KB, not MB!

Worth remembering: at least in the past, there was also an _upper_
limit to the size based on the lower-level bootblocks that read the
freebsd-boot partition, above which things would also break horribly.
I don't recall exactly what it was, but it was >512k and <1m (maybe
600k-ish?).  So don't just make it arbitrarily bigger.


-- 
Matthew Fuller     (MF4839)   |  fullermd at over-yonder.net
Systems/Network Administrator |  http://www.over-yonder.net/~fullermd/
           On the Internet, nobody can hear you scream.