manual: $(BR2_EXTERNAL)/{Config.in, external.mk} are mandatory

The manual is a bit vague about whether Config.in and external.mk
are mandatory or optional.

Make it explicit in the manual that they are mandatory.

Reported-by: Jérémy Rosen <jeremy.rosen@openwide.fr>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>