[initrd] Allow for images straddling the top of the reshuffle region

[initrd] Allow for images straddling the top of the reshuffle region

It is hypothetically possible for external heap memory allocated
during driver startup to have been freed before an image was
downloaded, which could therefore leave an image straddling the
address recorded as the top of the reshuffle region.

Allow for this possibility by skipping squashing for any images
already straddling (or touching) the top of the reshuffle region.

Signed-off-by: Michael Brown <mcb30@ipxe.org>