hw/arm/highbank: Fix resource leak and wrong image loading

Coverity reports a resource leak for sysboot_filename which is allocated
by qemu_find_file.

In addition, that name is used to get the size of the image, but a
different image name was used to load it.

In addition, instead of passing the maximum allowed image size the actual
image size was passed to load_image_targphys.

Fix all three issues.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Message-id: 1426326781-2488-1-git-send-email-sw@weilnetz.de
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
