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-host-tools.sh

@@ -2,5 +2,5 @@
 # vim: set sw=4 ts=4 sts=4 et :
 
 PORTAGE_CONFIGROOT="${CONFIGDIR:=${PWD}}"/portage/host \
-< "${CONFIGDIR}"/host-tools.packages xargs -ro \
+< host-tools.packages xargs -ro \
 emerge -vnuUj --rebuilt-binaries=y