For platforms such as the at91, the boot ROM imposes an upper limit
on barebox file size.  Prior to 5a1a5ed253 ("ARM: images: use piggydata"),
BAREBOX_MAX_PBLX_SIZE seems to have been the way to go for limiting
the size of the final barebox binary when using the PBL.
With pblx removed, this variable is of no use, so have the existing
BAREBOX_MAX_IMAGE_SIZE replace its functionality.

Currently BAREBOX_MAX_IMAGE_SIZE is only checked against in the non-PBL
case, so add a check in the PBL case as well.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>