ar71xx: Unifi AP Pro sysupgrade patch

The current implementation of mtd will not append the backup
file created by sysupgrade to the correct partition, as mtd will append
the data to first jffs2 partition it finds. As the kernel is also
stored on a jffs2 partition (which resides before the overlay
partition), the data will be appended to this partition.

To fix this problem, a new option

-s <number>             skip the first n bytes when appending data to the jffs2 partiton, defaults to "0"

is added to mtd.

Signed-off-by: Peter Wagner <tripolar@gmx.at>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38807 3c298f89-4303-0410-b956-a3cf2f4a3e73