Currently, postrm scripts are not being executed when package is removed
with APK since they are not being included at all.
So, lets use APK-s support for post-deinstall scripts and ship our postrm
scripts if packages define them.
Fixes: d788ab376f85 ("build: add APK package build capabilities")
Link: https://github.com/openwrt/openwrt/pull/17023
Signed-off-by: Robert Marko <robimarko@gmail.com>