Fix broken IB_STANDALONE option for OPKG due to an error in ifdef logic
where we weren't adding the required entry to reference the local files
in repositories.conf
Rework the ifdef to more explicit and restore original functionality of
this option.
While at it also provide different README for APK or OPKG.
Fixes: d788ab376f ("build: add APK package build capabilities")
Link: https://github.com/openwrt/openwrt/pull/15599
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
