core: get rid of our dummy br2-external tree

core: get rid of our dummy br2-external tree

Now that we generate a kconfig snippet, we can conditionally include the
BR2_EXTERNAL's Config.in only when BR2_EXTERNAL is supplied by the user,
which means our empty/dummy Config.in is no needed.

As for external.mk, we can also include it only when BR2_EXTERNAL is
supplied by the user, which means our empty/dummy external.mk is no
longer needed.

Ditch both of those files, and:

  - only generate actual content in the Kconfig snippet when we actually
    do have a BR2_EXTERNAL provided by the user (i.e. BR2_EXTERNAL is not
    empty);

  - add a variable that contains the path to the external.mk provided by
    the user, or empty if none, and include the path set in that variable
    (make can 'include' nothing without any problem! ;-) )

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>