core: add support for multiple br2-external trees

Currently, we only support at most one br2-external tree. Being able
to use more than one br2-external tree can be very useful.

A use-case would be for having a br2-external to contain the basic
packages, basic board defconfigs and board files, provided by one team
responsible for the "board-bringup", while other teams consume that
br2-external as a base, and complements it each with their own set of
packages, defconfigs and extra board files.

Another use-case would be for third-parties to provide their own
Buildroot packaging in a br2-external tree, along-side the archives for
their stuff.

Finally, another use-case is to be able to add FLOSS packages in a
br2-external tree, and proprietary packages in another. This allows
to not touch the Buildroot tree at all, and still be able to get in
compliance by providing only that br2-external tree(s) that contains
FLOSS packages, leaving aside the br2-external tree(s) with the
proprietary bits.

What we do is to treat BR2_EXTERNAL as a colon-separated (space-
separated also work, and we use that internally) list of paths, on which
we iterate to construct:

  - the list of all br2-external names, BR2_EXTERNAL_NAMES,

  - the per-br2-external tree BR2_EXTERNAL_$(NAME) variables, which
    point each to the actual location of the corresponding tree,

  - the list of paths to all the external.mk files, BR2_EXTERNAL_MKS,

  - the space-separated list of absolute paths to the external trees,
    BR2_EXTERNAL_DIRS.

Once we have all those variables, we replace references to BR2_EXTERNAL
with either one of those.

This cascades into how we display the list of defconfigs, so that it is
easy to see what br2-external tree provides what defconfigs. As
suggested by Arnout, tweak the comment from "User-provided configs" to
"External configs", on the assumption that some br2-external trees could
be provided by vendors, so not necessarily user-provided. Ditto the menu
in Kconfig, changed from "User-provided options" to "External options".

Now, when more than one br2-external tree is used, each gets its own
sub-menu in the "User-provided options" menu. The sub-menu is labelled
with that br2-external tree's name and the sub-menu's first item is a
comment with the path to that br2-external tree.

If there's only one br2-external tree, then there is no sub-menu; there
is a single comment that contains the name and path to the br2-external
tree.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Romain Naour <romain.naour@openwide.fr>
Cc: Julien CORJON <corjon.j@ecagroup.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

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