Patch to apply against HEAD of sys-app/portage project.
I did git clone git://git.overlays.gentoo.org/proj/portage.git on 2014-07-04
13:47:53.442947599 +0000.

diff -ru old_portage/pym/portage/const.py portage/pym/portage/const.py
--- old_portage/pym/portage/const.py	2014-07-04 16:35:37.462942289 +0200
+++ portage/pym/portage/const.py	2014-07-04 17:03:00.347420753 +0200
@@ -119,6 +119,7 @@
 	"other",
 )
 SUPPORTED_FEATURES       = frozenset([
+	"uncooperative-root",
 	"assume-digests",
 	"binpkg-logs",
 	"buildpkg",
diff -ru old_portage/cnf/make.conf.example portage/cnf/make.conf.example
--- old_portage/cnf/make.conf.example	2014-07-04 16:52:24.438859863 +0200
+++ portage/cnf/make.conf.example	2014-07-04 17:08:10.658545393 +0200
@@ -362,3 +362,29 @@
 #                               ${PACKAGE} - see description of PORTAGE_ELOG_COMMAND
 #                               ${HOST} - FQDN of the host portage is running on
 #PORTAGE_ELOG_MAILSUBJECT="[portage] ebuild log for \${PACKAGE} on \${HOST}"
+#
+# Variant for Prefix installations
+# ================================
+#
+#MAKEOPTS="-j2 TMPDIR=${EPREFIX}/tmp CONFIG_SHELL=${EPREFIX}/bin/sh SHELL=${EPREFIX}/bin/bash"
+#PORTAGE_PYTHON=$EPREFIX/usr/bin/python # e.g. for portage/bin/misc-functions.sh
+#PREFIX_DISABLE_GEN_USR_LDSCRIPT=set # e.g. for net-libs/libtirpc-0.2.2-r1.ebuild
+#FEATURES="uncooperative-root"
+#USE="uncooperative-root"
+# 
+#     has uncooperative-root ${FEATURES}
+# is a condition meaning that /tmp, /bin/bash,
+# /bin/sh, /dev, /usr/bin/{env,find,file} may not be available at buiding time
+# and despite being in various standards (FHS, ...) root administrator will not
+# cooperate or will break compatibility without notice. They are to be used now
+# in .eclass and in sys-app/portage (typical example: Android; needed for
+# gentooandroid.sourceforge.net).
+# It will likely be used in any *.ebuild file whose EAPI officially contains
+# SUPPORTED_FEATURES=uncooperative-root.
+# 
+#     use uncooperative-root
+# is the equivalent condition to use in files *.ebuild whose EAPI doesn't
+# officially contain SUPPORTED_FEATURES=uncooperative-root.  To enable that
+# condition, a file *.ebuild not inherite-ing anything should contain
+# IUSE=-uncooperative-root for that (otherwise, prefix.eclass will have
+# IUSE_IMPLICIT=uncooperative-root even if not directly inherited).
diff -ru old_portage/bin/ebuild-helpers/emake portage/bin/ebuild-helpers/emake
--- old_portage/bin/ebuild-helpers/emake	2014-07-04 16:17:50.847841909 +0200
+++ portage/bin/ebuild-helpers/emake	2014-07-04 16:30:26.020612108 +0200
@@ -11,6 +11,14 @@
 
 source "${PORTAGE_BIN_PATH:-/usr/lib/portage/bin}"/isolated-functions.sh
 
+#${MAKEOPTS} is not transmitted to things called by make
+has uncooperative-root ${FEATURES} && { 
+  export TMPDIR="${EPREFIX}"/tmp
+  export BASH="${EPREFIX}"/bin/bash
+  export CONFIG_SHELL="${EPREFIX}"/bin/sh
+}
+
+
 if [[ $PORTAGE_QUIET != 1 ]] ; then
 	(
 	for arg in ${MAKE:-make} $MAKEOPTS $EXTRA_EMAKE "$@" ; do
diff -ru old_portage/bin/ebuild.sh portage/bin/ebuild.sh
--- old_portage/bin/ebuild.sh	2014-07-04 15:59:24.022279274 +0200
+++ portage/bin/ebuild.sh	2014-07-04 16:01:08.613886159 +0200
@@ -90,8 +90,13 @@
 __qa_call() {
 	local shopts=$(shopt) OLDIFS="$IFS"
 	local retval
-	"$@"
-	retval=$?
+        if has uncooperative-root ${FEATURES} ; then
+		uncooperative-root-wrapper "$@" # from prefix.eclass
+		retval=$?
+	else
+		"$@"
+		retval=$?
+	fi
 	set +e
 	[[ $shopts != $(shopt) ]] &&
 		eqawarn "QA Notice: Global shell options changed and were not restored while calling '$*'"
diff -ru old_portage/bin/misc-functions.sh portage/bin/misc-functions.sh
--- old_portage/bin/misc-functions.sh	2014-07-04 16:03:59.416144645 +0200
+++ portage/bin/misc-functions.sh	2014-07-04 16:57:13.327124754 +0200
@@ -820,7 +820,7 @@
 	rm -f "${ED}"/usr/share/info/dir{,.gz,.bz2} || die "rm failed!"
 
 	if has multilib-strict ${FEATURES} && \
-	   [[ -x /usr/bin/file && -x /usr/bin/find ]] && \
+	   [[ -x "`has uncooperative-root ${FEATURES} && echo ${EPREFIX}`/usr/bin/file" && -x "`has uncooperative-root ${FEATURES} && echo ${EPREFIX}`/usr/bin/find" ]] && \
 	   [[ -n ${MULTILIB_STRICT_DIRS} && -n ${MULTILIB_STRICT_DENY} ]]
 	then
 		rm -f "${T}/multilib-strict.log"
@@ -1193,6 +1193,10 @@
 	fi
 	[ -n "${md5_hash}" ] && \
 		echo ${md5_hash} > "${PORTAGE_BUILDDIR}"/build-info/BINPKGMD5
+  	# inform about trial patching done by uncooperative-root
+  	has uncooperative-root ${FEATURES} && bzip2 -c \
+  	  < "${PORTAGE_BUILDDIR}"/temp/build.log \
+  	  > "${PORTAGE_BUILDDIR}"/build-info/build.log.bz2
 	__vecho ">>> Done."
 
 	# cleanup our temp tree
diff -ru old_portage/bin/phase-helpers.sh portage/bin/phase-helpers.sh
--- old_portage/bin/phase-helpers.sh	2014-07-04 16:27:01.655594020 +0200
+++ portage/bin/phase-helpers.sh	2014-07-04 16:27:22.888250815 +0200
@@ -471,6 +471,10 @@
 	local x
 	local pid=${BASHPID:-$(__bashpid)}
 
+	has uncooperative-root ${FEATURES} && { 
+		export TMPDIR="${EPREFIX}"/tmp
+		export CONFIG_SHELL="${EPREFIX}"/bin/sh
+	}
 	if ! ___eapi_has_prefix_variables; then
 		local EPREFIX=
 	fi
@@ -592,6 +596,7 @@
 
 einstall() {
 	# CONF_PREFIX is only set if they didn't pass in libdir above.
+	has uncooperative-root ${FEATURES} && export TMPDIR="${EPREFIX}"/tmp
 	local LOCAL_EXTRA_EINSTALL="${EXTRA_EINSTALL}"
 	if ! ___eapi_has_prefix_variables; then
 		local ED=${D}