pkgpriorities: generalize for any stage2

install2 and live are both stage2 flavours, and I see no reason
rescue (the last one) should be any different here.

features.in/pkgpriorities/install2/lib/50-pkgpriorities.mk

@@ -1,29 +0,0 @@
-# NB: A copy of ../../live/lib/50-aptprefs.mk.
-# TODO: Manage things to make duplicate files unnecessary.
-
-_IMAGE_APTBOX_ = $(WORKDIR)/chroot/$(WORKDIRNAME)/aptbox
-
-# Add prerequisite to the `build-image` target of
-# $(MKIMAGE_PREFIX)/targets.mk.
-build-image: $(_IMAGE_APTBOX_)/etc/apt/pkgpriorities
-
-_PINNED_PACKAGES_ = $(foreach pp,$(PINNED_PACKAGES),$(if $(findstring :,$(pp)),$(pp),$(pp):$(PIN_PRIORITY)))
-_PIN_PRIORITIES_ = $(sort $(foreach pp,$(_PINNED_PACKAGES_),$(lastword $(subst :, ,$(pp)))))
-_PKGPRIORITIES_ = $(subst \n ,\n,$(foreach pri,$(_PIN_PRIORITIES_),$(pri):$(patsubst %:$(pri),\n   %,$(filter %:$(pri),$(_PINNED_PACKAGES_)))\n))
-
-$(_IMAGE_APTBOX_)/etc/apt/pkgpriorities: prepare-image-workdir
-	@echo -e '$(_PKGPRIORITIES_)' | sed -e 's/[[:space:]]\+$$//' >$@
-	@if [ -s $@ ]; then \
-		if grep -q '^\(APT::\)\?Dir::Etc::pkgpriorities[[:space:]]' \
-			$(_IMAGE_APTBOX_)/etc/apt/apt.conf; \
-		then \
-			sed -i -e 's/\(Dir::Etc::pkgpriorities\)[[:space:]].*$$/\1 "$@";/g' \
-				$(_IMAGE_APTBOX_)/etc/apt/apt.conf; \
-		else \
-			echo 'Dir::Etc::pkgpriorities "$@";' \
-				>>$(_IMAGE_APTBOX_)/etc/apt/apt.conf; \
-		fi; \
-		echo "--- Package priorities ---" >&2; \
-		cat $@ >&2; \
-		echo "---" >&2; \
-	fi