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.

Makefile

@@ -5,12 +5,9 @@ update.tar: $(IMAGESDIR)/update.tar.zstd
 
 $(O)/.prepared: \
 		prepare.sh \
-		config-portage.sh \
 		setup-local-repo.sh \
-		$(shell find portage host-portage -type f) \
 		$(shell find patches/ebuilds -type f)
 	./prepare.sh
-	./config-portage.sh
 	./setup-local-repo.sh
 	mkdir -p $(O)
 	touch $(O)/.prepared