bootm: replace blob_start with image_start

For uImage always has a 64 bytes header, we couldn't expect to do
the xip from the header but should xip from the image start.

The latter logic in that section is also move the image from image_start
to the load address, so sync this logic to the xip operation.

Signed-off-by: Lei Wen <leiwen@marvell.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>