mkimage: Fix error message if write less data then expected

Add a new error message in case the size of data written
are shorter than the one expected.

Currently, it will lead to the following error message:

"mkimage: Write error on uImage: Success"

This is not explicit when the error is because the device
doesn't have enough space. Let's use a more understandable message:

"mkimage: Write only 4202432/4682240 bytes, probably no space left on the device"

Signed-off-by: Mylène Josserand <mylene.josserand@collabora.com>
Reviewed-by: Walter Lozano <walter.lozano@collabora.com>