portage: Add patch to filter EROOT from ebuild environment

EROOT must always be derived from ROOT, preserving it causes binary
packages to break in mysterious and often silent ways.

Upstream: https://bugs.gentoo.org/show_bug.cgi?id=490014

sdk_container/src/third_party/coreos-overlay/sys-apps/portage/files/portage-2.2.8-filter-eroot.patch

@@ -0,0 +1,35 @@
+From 6e0d2f0b0be100c033edf3593fe327bca061f0db Mon Sep 17 00:00:00 2001
+From: Michael Marineau <mike@marineau.org>
+Date: Sun, 20 Jul 2014 20:27:43 -0700
+Subject: [PATCH] environment: Filter EROOT for all EAPIs
+
+This variable is often defined in older EAPIs with "${EROOT:=$ROOT}"
+but it should never be preserved since ROOT may change. Bug #490014
+---
+ bin/phase-functions.sh | 9 ++++++---
+ 1 file changed, 6 insertions(+), 3 deletions(-)
+
+diff --git a/bin/phase-functions.sh b/bin/phase-functions.sh
+index f39a024..3a62904 100644
+--- a/bin/phase-functions.sh
++++ b/bin/phase-functions.sh
+@@ -101,10 +101,13 @@ __filter_readonly_variables() {
+ 	filtered_vars="$readonly_bash_vars $bash_misc_vars
+ 		$PORTAGE_READONLY_VARS $misc_garbage_vars"
+ 
+-	# Don't filter/interfere with prefix variables unless they are
+-	# supported by the current EAPI.
++	# Always filter EROOT to ensure it is redefined based on ROOT 
++	filtered_vars+=" EROOT"
++
++	# Don't filter/interfere with the other prefix variables unless
++	# they are supported by the current EAPI.
+ 	if ___eapi_has_prefix_variables; then
+-		filtered_vars+=" ED EPREFIX EROOT"
++		filtered_vars+=" ED EPREFIX"
+ 	fi
+ 
+ 	if has --filter-sandbox $* ; then
+-- 
+1.8.5.5
+

sdk_container/src/third_party/coreos-overlay/sys-apps/portage/portage-2.2.8-r3.ebuild

@@ -229,6 +229,7 @@ src_prepare() {
 		epatch "${WORKDIR}/${PN}-${PATCHVER}.patch"
 	fi
 	epatch "${FILESDIR}/${P}-add-disabled.patch"
+	epatch "${FILESDIR}/${P}-filter-eroot.patch"
 	einfo "Setting portage.VERSION to ${PVR} ..."
 	sed -e "s/^VERSION=.*/VERSION=\"${PVR}\"/" -i pym/portage/__init__.py || \
 		die "Failed to patch portage.VERSION"