sim: update ram load to use image size comparison

When building images for testing they are aligned to a flash write size,
which works fine for flash based comparisons, but since RAM is always
reset to 0, if the flash is erased to 0xff, the final bytes of the
alignment will differ even though they are not relevant (not part of the
image itself).

This commit adds a real image size parameter to the ImageData, so it can
be used by the RAM load to compare only the relevant bits of the image
(ignore the padding), and also updates the RAM test routine to use the
correct image size.

Signed-off-by: Fabio Utzig <utzig@apache.org>