docs/manual: add explanations about BR2_EXTERNAL

This commit updates the manual to add details on how to use the
BR2_EXTERNAL feature.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
manual: fix manual generation with BR2_EXTERNAL support

Reported-by: Ryan Barnett <rjbarnet@rockwellcollins.com>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Ryan Barnett <rjbarnet@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>