build-rootfs: Ignore USE changes for baselayout

We don't want to reinstall _baselayout_ when its USE flags change.
Indeed, USE flag changes are expected, because we first install it with
USE=build, but only temporarily.  Once the initial symlinks for
merged-usr are in place, it can be reinstalled without that flag.  We
don't need to keep reinstalling and toggling between states, so we
ignore USE flag changes for the first pass.

build-rootfs.sh

@@ -16,7 +16,7 @@ USE=build \
 emerge \
     --root=/mnt/gentoo \
     --config-root="${O}"/portage \
-    -kvnjU \
+    -kvnj \
     sys-apps/baselayout
 
 cat \