layerscape: fix platform_copy_config function

layerscape: fix platform_copy_config function

At this moment platform_copy_config function is used for every device
and function "export_partdevice" without "export_bootdevice" causes
multiple errors like that:
"sh: 1: unknown operand"

This patch fix usage of export_partdevice and split platform_copy_config
for sd-card images.

Fixes: 0841b68c91d3 ("layerscape: support sysupgrade for SD card ext4
rootfs")

Reviewed-by: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>