The __image_start and __image_end marker are used by barebox to
calculate the size of the barebox binary inside the RAM. Without the
definitions barebox calculates its own size to zero, which works until a
certain size is exceeded, than the PBL will place the binary lower into
memory than the calculation inside barebox.
Fix this by adding the appropriate image markers to the linker script.

Signed-off-by: Rouven Czerwinski <r.czerwinski@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>