luci-app-attendedsysugprade: allow to reinstall image

Now it's possible to re-install the currently running version in
advanced mode. This can be useful when installing packages via `opkg`
and then requesting the firmware with the packages stored in squashfs.

FIXES: https://github.com/openwrt/luci/issues/5809

Signed-off-by: Paul Spooren <mail@aparcar.org>