Check that u-boot.bin size looks correct

Check that the image size matches the size we get from u-boot.bin. If it
doesn't, that generally means that some extra sections are being added to
u-boot.bin, meaning that it is not possible to access data appended to
the U-Boot binary. This is used for device tree, so needs to work.

This problem was introduced by commit b02bfc4d. By adding a test we can
prevent a reccurence.

Signed-off-by: Simon Glass <sjg@chromium.org>