Not all architectures are referencing the image_end section from
code, in which case the linker is free to drop the section, which
then messes up the calculation of the _barebox_image_size linker
variable.

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