Separate base packages from customizations

The `build.packages` and `install.packages` files in the CONFIGDIR now
only need to include *additional* packages to install.  The packages
*required* for Aimee OS are always installed, listed in the
corresponding files in the source directory.

host-tools.packages

@@ -0,0 +1,9 @@
+sec-policy/selinux-base
+sys-apps/policycoreutils
+sys-apps/systemd
+sys-boot/grub
+sys-fs/btrfs-progs
+sys-fs/dosfstools
+sys-fs/genimage
+sys-fs/mtools
+sys-fs/squashfs-tools