For small systems we would put the zImage at 8MiB after
the start of memory, and put the DT a bit after the zImage.
When we encounter an image which is bigger than 8MiB
uncompressed, the kernel would try to relocate itself
and overwrite the DT.

Try to be more clever at zImage placement to avoid
triggering the kernel relocation.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>