imx6: add platform_pre_upgrade for sysupgrade

imx6: add platform_pre_upgrade for sysupgrade

We're in process of cleaning nand_do_platform_check. Currently is leaves
a special mark in /tmp/sysupgrade-nand-path triggering some diffent code
path in nand_upgrade_stage1. This can be a bit confusing.

The plan is to have the check function only check the image and nothing
else. Then platform code (platform_pre_upgrade) should trigger NAND
specific upgrade path. This is what this patch implements.

This follows ar71xx, lantiq and partially bcm53xx.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46944 3c298f89-4303-0410-b956-a3cf2f4a3e73