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.

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