Fix copyFromFile reading more data than requested

The copyFromFile buffer function will always attempt to fill the full
temporary buffer when copying from the file. This is an issue because
the file might contain data for other buffers as well.

To resolve this, the function will now take the remaining size to be
read into account and not read more than that from the file.

Change-Id: I0e9265d6f203e4157c5d40607bb522cbf457a34a
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>