include/rootfs: rework handling of post-install scripts for APK

include/rootfs: rework handling of post-install scripts for APK

Rework handling of post-install scripts for APK. As we do with OPKG,
lets just iterate between each post-install package so we can actually
check if something fail in applying them.

To do this we first extract each .post-install script in APK
scripts.tar.

Also remove these files from final image as they are needed only for the
first installation of the packages.

Link: https://github.com/openwrt/openwrt/pull/15543
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>