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/chroot-grep/template

@@ -6,7 +6,7 @@
 unset run_depends build_depends revision
 
 revision=2
-base_chroot=yes
+bootstrap=yes
 pkgname=chroot-grep
 wrksrc=grep-${version}
 configure_args="--disable-perl-regexp --disable-nls ac_cv_path_GREP=grep"