build: Fix building binpkgs for BDEPENDs

Apparently `FEATURES` and/or `EMERGE_DEFAULT_OPTS` set in `make.conf` do
not seem to apply to packages pulled in as `BDEPEND` dependencies and
installed in `/` instead of `/usr/${target}`.  This causes binary
packages to not be built/used for those packages.  Passing `--buildpkg`
(`-b`) and `--usepkg` (`-k`) on the command line ensures that binary
packages are built for everything.

rebuild-pkg.sh

@@ -9,4 +9,4 @@ unset MAKEFLAGS MAKEOVERRIDES MAKELEVEL
 
 export PORTAGE_CONFIGROOT="${CONFIGDIR}"/portage/target
 
-${target}-emerge -bv1j --usepkg-exclude="$*" "$@"
+${target}-emerge -bkv1j --usepkg-exclude="$*" "$@"