xbps-src: support building bootstrap pkgs in chroot with -D.

The base_chroot var has been renamed to bootstrap, to avoid conflicts
with the base-chroot pkg. So now you can rebuild any bootstrap package
if the masterdir is ready (base-chroot already) installed and
installation is only to destdir.

srcpkgs/bash/template

@@ -34,7 +34,7 @@ long_desc="
  to sixty-four. In addition, most sh scripts can be run by Bash without
  modification."
 
-base_chroot=yes
+bootstrap=yes
 register_shell="/bin/bash"
 
 Add_dependency run glibc