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>

docs/manual/adding-packages-directory.txt

@@ -298,6 +298,16 @@ management, use this format:
 foo needs udev /dev management and a toolchain w/ featA, featB, featC
 --------------------------
 
+==== Dependencies on features provided by virtual packages
+
+Some features can be provided by more than one package, such as the
+openGL libraries.
+
+See xref:virtual-package-tutorial[] for more on the virtual packages.
+
+See xref:virtual-package-list[] for the symbols to depend on if your package
+depends on a feature provided by a virtual package.
+
 === The +.mk+ file
 
 [[adding-packages-mk]]

docs/manual/adding-packages-virtual.txt

@@ -83,6 +83,9 @@ On line 3, we select +BR2_PACKAGE_HAS_SOMETHING_VIRTUAL+, and on line 11, we
 set the value of +BR2_PACKAGE_PROVIDES_SOMETHING_VIRTUAL+ to the name of the
 provider, but only if it is selected.
 
+See xref:virtual-package-list[] for the symbols to select if you implement
+a new provider for an existing virtual package.
+
 ==== Provider's +.mk+ file
 
 The +.mk+ file should also declare an additional variable
@@ -96,6 +99,9 @@ packages it is an implementation of:
 Of course, do not forget to add the proper build and runtime dependencies for
 this package!
 
+See xref:virtual-package-list[] for the names of virtual packages to provide
+if you implement a new provider for an existing virtual package.
+
 ==== Notes on depending on a virtual package
 
 When adding a package that requires a certain +FEATURE+ provided by a virtual

docs/manual/appendix.txt

@@ -14,6 +14,14 @@ include::makeusers-syntax.txt[]
 
 include::package-list.txt[]
 
+[[virtual-package-list]]
+== List of virtual packages
+
+These are the virtual packages known to +Buildroot+, with the
+corresponding symbols and providers.
+
+include::virtual-package-list.txt[]
+
 [[host-package-list]]
 == List of host utilities available in Buildroot