ARM: i.MX: external NAND boot: use image size from image header
When compiling with multiimage support ld_var(_barebox_image_size) only
contains the length of the PBL image, but not including the appended
compressed data. With this patch the image size is read from the barebox
header instead which contains the correct size, either from the linker
or from the fix_size tool.
This makes the external_nand_boot compatible with multiimage support.
Tested on Phytec phyCARD-i.MX27 with and without PBL.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>