build: Install util-linux from binpkg if possible

_sys-apps/util-linux_ first must be built (in the prepare stage) with
`USE=build` and then reinstalled later without that flag.  We can save
time in the second step by using binary packages if they are already
available (i.e. from a previous build).

build.sh

@@ -9,7 +9,7 @@ unset MAKEFLAGS MAKEOVERRIDES MAKELEVEL
 
 export PORTAGE_CONFIGROOT="${O}"/portage
 
-${target}-emerge -vuUDj sys-apps/util-linux
+${target}-emerge -bkvuUDj sys-apps/util-linux
 
 cat \
     build.packages \