Reinstall packages for updates/changed USE flags

Packages which have been updated and/or had USE flag changes will now be
reinstalled in the destination root directory automatically.

build-rootfs.sh

@@ -25,12 +25,12 @@ cp -r portage/. /mnt/gentoo/etc/portage/
 ROOT=/mnt/gentoo \
 PORTAGE_CONFIGROOT="${PWD}"/portage/config/target \
 < install.packages xargs -ro \
-${target}-emerge -Kvnj --rebuilt-binaries=y
+${target}-emerge -KvnuUDj --rebuilt-binaries=y
 
 ROOT=/mnt/gentoo \
 PORTAGE_CONFIGROOT="${PWD}"/portage/config/target \
 < installonly.packages xargs -ro \
-${target}-emerge -vnj
+${target}-emerge -vnuUDj
 
 ROOT=/mnt/gentoo \
 locale-gen