tplink-safeloader: add support to convert factory into sysupgrade

Those converted factory images can be used to regain the original
tp-link firmware.
Be aware of firmware upgrade which additional require changes of
other partition than os-image (kernel) & file-system (rootfs).
OEM factory images from tplink can change nearly all partitions.
However using those images, OpenWrt's sysupgrade will only
modify the partitions os-image and file-system.

Signed-off-by: Alexander Couzens <lynxis@fe80.eu>