docs/manual: add the virtual packages list

Add the list of virtual packages as an appendix to the manual.

Also reference this list from appropriate locations elsewhere in
the manual:

  - in section 7.2.2. "Config.in file", after the existing explanations
    on dependencies on target and toolchain options, on a linux kernel,
    and on udev /dev management,

  - in section 7.2.10. "Infrastructure for virtual packages", in the
    provider Config.in and .mk explanations, to have the list of existing
    symbols to select (in Config.in) and packages to provide (in .mk).

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