Make more project config files optional

`build.packages`, `installonly.packages`, and `busybox.symlinks` are now
optional in the `$CONFIGDIR`.  If they do exist, they are combined with
the same files in the Aimee OS source.

Makefile

@@ -20,7 +20,7 @@ $(O)/.built: \
 		build.sh \
 		build.packages \
 		install.packages \
-		$(CONFIGDIR)/build.packages \
+		$(wildcard $(CONFIGDIR)/build.packages) \
 		$(CONFIGDIR)/install.packages \
 		$(shell find $(CONFIGDIR)/portage/target -type f) \
 		$(O)/.prepared
@@ -38,8 +38,8 @@ $(O)/.ready: \
 		build-rootfs.sh \
 		install.packages \
 		$(CONFIGDIR)/install.packages \
-		$(CONFIGDIR)/installonly.packages \
-		$(CONFIGDIR)/busybox.symlinks \
+		$(wildcard $(CONFIGDIR)/installonly.packages) \
+		$(wildcard $(CONFIGDIR)/busybox.symlinks) \
 		$(O)/linux/arch/arm64/boot/Image.gz \
 		$(O)/semanage.mods \
 		$(shell find overlay -type f) \