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>

target/imagebuilder/files/Makefile

@@ -220,4 +220,12 @@ manifest: FORCE
 		$(if $(PROFILE),USER_PROFILE="$(PROFILE_FILTER)") \
 		$(if $(PACKAGES),USER_PACKAGES="$(PACKAGES)"))
 
-.SILENT: help info image manifest
+whatdepends: FORCE
+ifeq ($(PACKAGE),)
+	@echo 'Variable `PACKAGE` is not set but required by `whatdepends`'
+	@exit 1
+endif
+	@$(MAKE) -s package_reload
+	@$(OPKG) whatdepends -A $(PACKAGE)
+
+.SILENT: help info image manifest whatdepends