sys-apps/config-site: Add from Gentoo

It's from Gentoo commit 940fa400354017ec9cc840f3e2cb897f73bce9c6.

sdk_container/src/third_party/portage-stable/sys-apps/config-site/config-site-0.ebuild

@@ -0,0 +1,24 @@
+# Copyright 2023 Gentoo Authors
+# Distributed under the terms of the GNU General Public License v2
+
+EAPI=8
+
+DESCRIPTION="config.site to load dropins from config.site.d"
+HOMEPAGE="https://wiki.gentoo.org/wiki/Project:Base"
+S=${WORKDIR}
+
+LICENSE="GPL-2"
+SLOT="0"
+KEYWORDS="~alpha amd64 arm arm64 hppa ~ia64 ~loong ~m68k ~mips ppc ppc64 ~riscv ~s390 sparc x86"
+
+RDEPEND="!<sys-devel/crossdev-20230209-r1"
+
+src_configure() {
+	sed -e "s|@datarootdir@|${EPREFIX}/usr/share|" \
+		"${FILESDIR}/config.site.in" > config.site || die
+}
+
+src_install() {
+	insinto /usr/share
+	doins config.site
+}

sdk_container/src/third_party/portage-stable/sys-apps/config-site/files/config.site.in

@@ -0,0 +1,10 @@
+# Copyright 2023 Gentoo Authors
+# Distributed under the terms of the GNU General Public License v2
+
+for script in "@datarootdir@/config.site.d"/*.conf; do
+	test -f "${script}" || continue
+	test -r "${script}" || continue
+	echo "${0##*/}: loading site script ${script}"
+	. "${script}" || break
+done
+unset script

sdk_container/src/third_party/portage-stable/sys-apps/config-site/metadata.xml

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE pkgmetadata SYSTEM "https://www.gentoo.org/dtd/metadata.dtd">
+<pkgmetadata>
+	<maintainer type="project">
+		<email>base-system@gentoo.org</email>
+	</maintainer>
+</pkgmetadata>