buildroot

docs/manual: document the br2-external desc: field

Docuement the new, optional desc: field for an external.desc file.

That part of the manual was starting to be a bit of a mess, so
reorganise it. Provide a complete br2-external tree example.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>