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>

docs/manual/customize-outside-br.txt

@@ -83,6 +83,7 @@ Or disable the usage of external definitions:
    logic. Buildroot automatically includes +BR2_EXTERNAL/Config.in+ to
    make it appear in the top-level configuration menu, and includes
    +BR2_EXTERNAL/external.mk+ with the rest of the makefile logic.
+   Providing those two files is mandatory, but they can be empty.
 +
 The main usage of this is to store package recipes. The recommended
    way to do this is to write a +BR2_EXTERNAL/Config.in+ that looks