Simplify specifying packages to build/install

The Portage packages that need to be built and/or installed are now
specified in the `build.packages` and `install.packages` files,
respectively.  Similarly, packages to be installed on the host system
are specified in `host-tools.packages`.  Finally, the
`installonly.packages` file contains a list of packages that are
installed in the destination root, but not built in the sysroot
beforehand.

This allows `make` to better track when the package sets change.  It
will also make it easier to maintain different sets for different
variants in the future.

Makefile

@@ -19,6 +19,7 @@ prepare: $(O)/.prepared
 
 $(O)/.host-tools: \
 		build-host-tools.sh \
+		host-tools.packages \
 		$(O)/.prepared
 	./build-host-tools.sh
 	touch $(O)/.host-tools
@@ -27,18 +28,27 @@ host-tools: $(O)/.host-tools
 
 $(O)/.built: \
 		build.sh \
-		build-rootfs.sh \
-		$(O)/linux/arch/arm64/boot/Image.gz \
+		build.packages \
+		install.packages \
 		$(O)/.host-tools \
 		$(O)/.prepared
 	./build.sh
-	./build-rootfs.sh
 	touch $(O)/.built
 
+$(O)/.ready: \
+		build-rootfs.sh \
+		install.packages \
+		installonly.packages \
+		$(O)/linux/arch/arm64/boot/Image.gz \
+		$(O)/.host-tools \
+		$(O)/.built
+	./build-rootfs.sh
+	touch $(O)/.ready
+
 $(IMAGESDIR)/rootfs.squashfs: \
 		build-squashfs.sh \
 		squashfs.exclude \
-		$(O)/.built
+		$(O)/.ready
 	./build-squashfs.sh "$(IMAGESDIR)"
 
 squashfs: $(IMAGESDIR)/rootfs.squashfs