ARM64: move to GPT scheme in sysinstall

It's time to abandon MBR installations on
     ARM64 platforms.

     Obtained from:         Semihalf
     Submitted by:          Dominik Ermel <der@semihalf.com>
     Sponsored by:          Cavium
     Reviewed by:           gjb, emaste, marcel
     Approved by:           re (gjb)
     Differential Revision: https://reviews.freebsd.org/D6798

release/arm64/make-memstick.sh

@@ -38,6 +38,6 @@ fi
 rm ${1}/etc/fstab
 rm ${1}/etc/rc.conf.local
 
-mkimg -s mbr -p efi:=${1}/boot/boot1.efifat -p freebsd:=${2}.part -o ${2}
+mkimg -s gpt -p efi:=${1}/boot/boot1.efifat -p freebsd:=${2}.part -o ${2}
 rm ${2}.part