arm64: set the correct partition type in make-memstick.sh

[mhorne]

Summary

We create a UFS root filesystem using makefs(8), and later pass it to mkimg(1) when
creating the final image. The correct partition type is freebsd-ufs; the freebsd
parition type is for partitions containing a BSD disklabel.

Test Plan

With this change:

# gpart show
=>     3  672344  vtbd0  GPT  (328M)
       3   66584      1  efi  (33M)
   66587  605760      2  freebsd-ufs  (296M)

[emaste]

This is just the partition type reported in the gpt header is it not?
i.e. there was not actually a disklabel before

[mhorne]

In D26987#602005, @emaste wrote:

This is just the partition type reported in the gpt header is it not?
i.e. there was not actually a disklabel before

Ahh, this highlights the gap in my understanding. I was unsure if -p freebsd:= actually created the disklabel or not, but upon another reading it clearly just sets the partition type, with whatever contents you pass to it.

So yes, that is what this change actually does. I will reword the description.