Image size: should use GB in place of GiB

[ocochard]

I've just create an "-s 16" image with build-arm-image.sh: The final image has the size of 16 GiB and didn't fit on my 16GB SD card (16GB=14.9GiB).
nanobsd script take care of this behavior.

[bitboss-ca]

Yes. What I do is follow the steps in https://github.com/daveish/freebsd-arm-tools#determining-your-sd-card-size to determine my card size in MB and then specify that. Add M to the size var to specify megabytes instead of the default unit of GB. Although, you are right, this function should be built in.