archiso/mkarchiso: create an empty /etc/machine-id

Fixes archlinux/archiso#42 .

archiso/mkarchiso

@@ -204,6 +204,8 @@ _cleanup () {
     # Delete package pacman related files.
     find "${work_dir}" \( -name '*.pacnew' -o -name '*.pacsave' -o -name '*.pacorig' \) -delete
     _msg_info "Done!"
+    # Create an empty /etc/machine-id
+    printf '' > "${work_dir}/airootfs/etc/machine-id"
 }
 
 # Makes a ext4 filesystem inside a SquashFS from a source directory.