docs/manual: add explanations about BR2_EXTERNAL

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

[Peter: minor tweaks, fix asciidoc tag as pointed out by Samuel]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: "Samuel Martin" <s.martin49@gmail.com>
Acked-by: Ryan Barnett <rjbarnet@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>