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.

build.sh

@@ -12,6 +12,8 @@ export PORTAGE_CONFIGROOT="${CONFIGDIR}"/portage/target
 ${target}-emerge -vuUDj sys-apps/util-linux
 
 cat \
+    build.packages \
+    install.packages \
     "${CONFIGDIR}"/build.packages \
     "${CONFIGDIR}"/install.packages \
     | xargs -ro \