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) \

build-rootfs.sh

@@ -33,11 +33,13 @@ ${target}-emerge \
     -KvnuUDj \
     --rebuilt-binaries=y
 
+if [ -f "${CONFIGDIR}"/installonly.packages ]; then
     < "${CONFIGDIR}"/installonly.packages xargs -ro \
     ${target}-emerge \
         --root=/mnt/gentoo \
         --config-root="${CONFIGDIR}"/portage/target \
         -vnuUDj
+fi
 
 ROOT=/mnt/gentoo \
 locale-gen
@@ -52,13 +54,16 @@ mkdir -p \
 
 : > /mnt/gentoo/etc/machine-id
 
-while read name; do
+cat \
+    "${CONFIGDIR}"/busybox.symlinks \
+    busybox.symlinks \
+| while read name; do
     if [ ! -h /mnt/gentoo/bin/"${name}" ]; then
         printf "'/bin/%s' -> 'busybox'\n" "${name}"
         ln -snf busybox /mnt/gentoo/bin/"${name}" \
             || printf 'Failed to create busybox symlink for %s\n' "${name}"
     fi
-done < "${CONFIGDIR}"/busybox.symlinks
+done
 
 rsync -rltpDO overlay/ /mnt/gentoo/
 if [ -d "${CONFIGDIR}"/overlay ]; then

busybox.symlinks

@@ -0,0 +1,62 @@
+basename
+cat
+chcon
+cp
+dd
+df
+du
+echo
+env
+false
+free
+fsync
+fuser
+grep
+groups
+head
+id
+install
+ip
+kill
+less
+link
+ln
+ls
+lsof
+mkdir
+mkfifo
+mktemp
+mv
+nice
+nproc
+pgrep
+ping
+ping6
+pkill
+ppidof
+printf
+ps
+readahead
+readlink
+realpath
+rm
+rmdir
+sed
+seq
+sh
+sha1sum
+sha256sum
+sha3sum
+sha512sum
+sort
+stat
+stty
+sync
+tac
+tail
+tar
+tee
+truncate
+tty
+uname
+uptime