Always add our apt_preference(5) pinning stanza when using local-packages.

This mostly reverts my most recent chroot_local-packages/preferences commit
as I found problems when re-running parts of the build: as the pinning
stanza was removed, calls to "apt-get upgrade" from within live-helper
would remove replacement packages placed in chroot_local-packages.

This isn't 100% foolproof - running apt-get upgrade on the binary image may
still attempt to remove replacement packages (heavy users of this should
probably be using their own repo for this sort of thing anyway).

helpers/lh_chroot_apt

@@ -159,3 +159,13 @@ case "${1}" in
 		Usage
 		;;
 esac
+
+if ls config/chroot_local-packages/*.deb > /dev/null 2>&1
+then
+	cat >>chroot/etc/apt/preferences <<EOF
+
+Package: *
+Pin: release o=chroot_local-packages
+Pin-Priority: 1001
+EOF
+fi

helpers/lh_chroot_install-packages

@@ -47,32 +47,19 @@ then
 	# Restoring cache
 	Restore_cache cache/packages_chroot
 
-	# Ensure chroot_local-packages override all other packages
-	if [ -e chroot/etc/apt/preferences ]
-	then
-		cp chroot/etc/apt/preferences chroot/etc/apt/preferences.live
-	fi
-
-	cat >> chroot/etc/apt/preferences.live <<EOF
-Package: *
-Pin: release o=chroot_local-packages
-Pin-Priority: 1001
-EOF
-
 	# Installing packages
 	case "${LH_APT}" in
 		apt|apt-get)
-			Chroot "xargs --arg-file=/root/chroot_packages apt-get ${APT_OPTIONS} -o Dir::Etc::preferences=preferences.live install"
+			Chroot "xargs --arg-file=/root/chroot_packages apt-get ${APT_OPTIONS} install"
 			;;
 
 		aptitude)
-			Chroot "xargs --arg-file=/root/chroot_packages aptitude ${APTITUDE_OPTIONS} -o Dir::Etc::preferences=preferences.live install"
+			Chroot "xargs --arg-file=/root/chroot_packages aptitude ${APTITUDE_OPTIONS} install"
 			;;
 	esac
 
 	# Tidying up
 	rm -f chroot/root/chroot_packages
-	rm -f chroot/etc/apt/preferences.live
 
 	# Saving cache
 	Save_cache cache/packages_chroot