Use Portage config directly from src dir

Instead of copying the Portage configuration files to `/etc/portage` and
`/usr/${target}/etc/portage`, the build scripts now use the
configuration directories from the source directory.  This avoids issues
with changes (especially removal of files) getting propagated to the
actual configuration paths.

build.sh

@@ -7,6 +7,8 @@ set -e
 
 unset MAKEFLAGS MAKEOVERRIDES MAKELEVEL
 
+export PORTAGE_CONFIGROOT="${PWD}"/portage/config/target
+
 ${target}-emerge -vuUDj sys-apps/util-linux
 
 cat \