When an image is loaded to SRAM we can normally only load a part of the
full image, limited by the available space in SRAM. We currently load
the pblb image which is the executable part of the PBL without the
compressed barebox payload. We are going to link the compressed barebox
image into the pbl image though, so we can't see the desired load size
from outside the image anymore.

This adds a max_load_size option to set this size explicitly.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>