build: add whatdepends target to imagebuilder

build: add whatdepends target to imagebuilder

The package manager `opkg` offers the function `whatdepends` to print
packages that depend on a specific package.

This feature is useful when used in a CI to not only build an upgraded
package but all packages with a dependency.

Usage:
    make whatdepends PACKAGE=libipset

The resulting list can be fed into a SDK building all packages and warn
if anything fails.

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