support/scripts: generate a list of virtual packages

Generate an asciidoc table that can be included in the manual, that
lists the existing virtual packages, the corresponding symbols, and
their providers (and sub-options thereof).

The core of this change is the addition of a new formatter for virtual
packages. This formatter is a bit tricky, as it has to catter for a
bunch of corner cases:
  - provider is not a package, but is sub-options of a package
  - such a sub-option may be itself 'select'-ed by one or more
    other sub-options
  - legacy packages should not be considered as a provider

Those cases are real:
  - sub-options of mesa3d provide EGL or GLES
  - selected sub-options of mesa3d provide GL
  - udev is a legacy package, but it provides udev

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>