imagebuilder: fix OPKG package_whatdepends support

imagebuilder: fix OPKG package_whatdepends support

Trying to use 'package_whatdepends' feature of the ImageBuilder with OPKG
will currently fail as OPKG does not support "list --depends" call at all,
it seems that this is a mixup from the original APK support commit.

So, lets restore 'package_whatdepends' support for OPKG by calling
"whatdepends -A" instead as we used to before APK support.

Fixes: d788ab376f85 ("build: add APK package build capabilities")
Link: https://github.com/openwrt/openwrt/pull/17022
Signed-off-by: Robert Marko <robimarko@gmail.com>
(cherry picked from commit 52519a59a859d349f154ed79e0a3d63457b6aa6a)