New package :step 'bootstrap', this step happens before 'pre' packages.
A new layer names 'spacemacs-bootstrap' gather all the ':step bootstrap'
packages. This layer is special and is always the first element of
the variable 'configuration-layer--layers' which assure that all
bootstrap packages are configured first.
This new layer leverages the configuration layer system, removes the
clutter of package installations in the function 'spacemacs/init' and
isolate the bootstrap packages in one place.
