sys-libs/glibc: Apply Flatcar modifications

- take care of nscd.conf via tmpfiles, add files/nscd-conf.tmpfiles.
  - comment out 'dostrip -x' to force the OS image binaries to be stripped
  - remove everything glibc wants to put under /etc since we use
    baselayout to provide that

sdk_container/src/third_party/coreos-overlay/sys-libs/glibc/README.md

@@ -0,0 +1,9 @@
+# GLibc
+
+The system's C library, sometimes referred to as "service pack for the C
+language". The build recipe has a single modification over the one Gentoo
+upstream uses: in the installation callback `glibc_do_src_install`, we remove
+all of glibc's `/etc` files right after the stock glibc build diligently
+installed them, since we ship our own `/etc` stuff via the `baseimage` recipe.
+The addition sits at the end of the `glibc_do_src_install` function and is duly
+labelled `## Flatcar Container Linux: ...`.

sdk_container/src/third_party/coreos-overlay/sys-libs/glibc/files/nscd-conf.tmpfiles

@@ -0,0 +1,2 @@
+L /etc/nscd.conf	-	-	-	-	../usr/share/baselayout/nscd.conf
+d /var/db/nscd		-	-	-	-	-

sdk_container/src/third_party/coreos-overlay/sys-libs/glibc/glibc-2.36-r5.ebuild

@@ -1290,12 +1290,13 @@ glibc_do_src_install() {
 	# '#define VERSION "2.26.90"' -> '2.26.90'
 	local upstream_pv=$(sed -n -r 's/#define VERSION "(.*)"/\1/p' "${S}"/version.h)
 
+	# Flatcar: override this and strip everything to keep image size at bay
 	# Avoid stripping binaries not targeted by ${CHOST}. Or else
 	# ${CHOST}-strip would break binaries build for ${CTARGET}.
-	is_crosscompile && dostrip -x /
+	# is_crosscompile && dostrip -x /
 	# gdb thread introspection relies on local libpthreas symbols. stripping breaks it
 	# See Note [Disable automatic stripping]
-	dostrip -x $(alt_libdir)/libpthread-${upstream_pv}.so
+	# dostrip -x $(alt_libdir)/libpthread-${upstream_pv}.so
 
 	if [[ -e ${ED}/$(alt_usrlibdir)/libm-${upstream_pv}.a ]] ; then
 		# Move versioned .a file out of libdir to evade portage QA checks
@@ -1478,6 +1479,23 @@ glibc_do_src_install() {
 		run_locale_gen --inplace-glibc "${ED}/"
 		sed -e 's:COMPILED_LOCALES="":COMPILED_LOCALES="1":' -i "${ED}"/usr/sbin/locale-gen || die
 	fi
+
+	## Flatcar Container Linux: Add some local changes:
+	# - Config files are installed by baselayout, not glibc.
+	# - Install nscd/systemd stuff in /usr.
+
+	# Use tmpfiles to put nscd.conf in /etc and create directories.
+	insinto /usr/share/baselayout
+	if ! in_iuse nscd || use nscd ; then
+		doins "${S}"/nscd/nscd.conf || die
+		newtmpfiles "${FILESDIR}"/nscd-conf.tmpfiles nscd-conf.conf || die
+	fi
+
+	# Clean out any default configs.
+	rm -rf "${ED}"/etc
+
+	# Restore this one for the SDK.
+	test ! -e "${T}"/00glibc || doenvd "${T}"/00glibc
 }
 
 glibc_headers_install() {