lantiq: add platform_pre_upgrade for sysupgrade

lantiq: add platform_pre_upgrade for sysupgrade

As explained in recent sysupgrade/nand.sh commits, current NAND
sysupgrade is a bit misleading because of nand_do_platform_check
behavior. It leaves a special mark in /tmp/sysupgrade-nand-path
triggering some diffent code path in nand_upgrade_stage1.

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.

Please note that because of current nand_upgrade_stage1 implementation
this patch doesn't change any behaior yet. It only prepares lantiq
target for changing nand_do_platform_check (it will be possible after
preparing all other targets as well).

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46939 3c298f89-4303-0410-b956-a3cf2f4a3e73