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support/scripts: introduce a symbol formatter to generate package lists

Currently, we can generate two different tables of packages:
  - a single-column table with the symbols' prompts,
  - a two-column table with the symbols' prompts and locations in the
    menuconfig.

For virtual packages, this is not enough, since we will have to display
more columns, with different content:
  - the virtual package name (but such symbols do not have a prompt)
  - the symbol name
  - the providers for the virtual package

So, instead of having a single function that knows how to generate any
table, introduce a formatter function that is passed as argument to,
and called by format_asciidoc_table(). Such formatter functions are
responsible for providing:
  - the layout of the table (number of columns, column arrangement),
  - the formatted header line,
  - a formatted line for a symbol.

What the formatter should ouput depends on its arguments:
  - if none are passed, the layout is returned,
  - if the header label is passed, it returns the formatted header line,
  - otherwise, it returns the formatted line for a symbol.

Two formatter functions are introduced in this changeset, to replace the
current 'sub_menu' feature:
  - _format_symbol_prompt() to display a one-column table with only the
    symbols' prompts,
  - _format_symbol_prompt_location() to display a two-column table with
    the symbols' prompts and locations.

This will help us to later introduce a new formatter to generate a table
for virtual packages.

[Thanks to Samuel for his pythonistic help!]

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>