Make copytoram automatic

Currently, copytoram=y needs to be explicitly enabled, but that's tedious and having additional boot loader entries just bloats the boot loader menu.

I think that copytoram should be automatically enabled if there's enough free RAM.

For example, if the free RAM is more than the rootfs image size + 2 GiB:

if [ "$(awk '$1 == "MemFree:" { print $2 }' /proc/meminfo)" -gt $(( $(du -bsk "$fs_img" | cut -f 1) + 2097152 )) ]; then
  copytoram="y"
fi