prepare: Fix binpkg build for libcap

Since we have to build *sys-libs/libcap* with the default Portage
configuration in order to avoid the circular dependency with PAM,
our configuration for binary package builds is not yet in place.  We
need to explicitly specify where to put the built packages and enable
multi-instance packages.

prepare.sh

@@ -14,13 +14,16 @@ chown portage: "$(portageq envvar DISTDIR)"
 
 ARCH=$(PORTAGE_CONFIGROOT=/usr/${target} portageq envvar ARCH)
 
-
+if [ -f /usr/${target}/etc/portage/make.conf ]; then
+    sed -i '/PKGDIR=/d' /usr/${target}/etc/portage/make.conf
+fi
 # For some reason, libcap installs its pkg-config files in the wrong
 # place.  More strangely, even though `PKG_CONFIG_PATH` contains that
 # directory, `pkg-config` does not find the `.pc` files for libcap.
 # We work around this by merging /usr/lib64/pkgconfig and
 # /usr/lib/pkgconfig.
-${target}-emerge -bk1vnj sys-libs/libcap
+FEATURES="${FEATURES} binpkg-multi-instance" \
+${target}-emerge -kb1vnj sys-libs/libcap
 if [ -d /usr/${target}/usr/lib/pkgconfig ] \
         && [ ! -d /usr/${target}/usr/lib64/pkgconfig ]; then
     mv /usr/${target}/usr/lib/pkgconfig /usr/${target}/usr/lib64