docs/manual: document the br2-external NAME

Update the manual with the new external.desc mandatory file.

Take the opportunity to add a section listing all mandatory files,
Config.in, external.mk and the new external.desc, instead of just
hinting about them in the external package recipes section.

Change the examples to use the NAME-suffixed variable instead of the
raw BR2_EXTERNAL variable.

Change all references to BR2_EXTERNAL elsewhere in the manual to now
use the 'br2-external tree' terminology.

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: Samuel Martin <s.martin49@gmail.com>
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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