Introduce $install_priority for use in templates.

A higher number in this var will make the binary package to
be installed before other with lower priority. By default, all
packages have a 0 priority. xbps-base-dirs uses 100, because it
needs to be one of the first installed packages.

--HG--
extra : convert_revision : 81a88b676d2ae3180914612371619f19e1765d02

templates/xbps-base-dirs.tmpl

@@ -8,6 +8,7 @@ long_desc="
  This package installs the base system directories."
 
 base_chroot=yes
+install_priority=100
 
 do_install()
 {