rebuild-pkg: Script to rebuild/reinstall a binpkg

This script can be used to rebuild a binary package in the SYSROOT and
reinstall it in the destination root.

```sh
./rebuild-pkg sec-policy/selinux-aimee-os
make -W /tmp/build/.built O=/tmp/build IMAGESDIR=${PWD}/images
```

rebuild-pkg.sh

@@ -0,0 +1,12 @@
+#!/bin/sh
+# vim: set sw=4 ts=4 sts=4 et :
+
+set -e
+
+. ./config
+
+unset MAKEFLAGS MAKEOVERRIDES MAKELEVEL
+
+export PORTAGE_CONFIGROOT="${PWD}"/portage/config/target
+
+${target}-emerge -bv1j --usepkg-exclude="$*" "$@"