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

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.

Reviewed by:	emaste
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D26987

release/arm64/make-memstick.sh

@@ -45,7 +45,7 @@ make_esp_file ${espfilename} ${fat32min} ${1}/boot/loader.efi
 
 mkimg -s gpt \
     -p efi:=${espfilename} \
-    -p freebsd:=${2}.part \
+    -p freebsd-ufs:=${2}.part \
     -o ${2}
 rm ${espfilename}
 rm ${2}.part