[Buildroot] [PATCH 20/21 v2] docs/manual: document the br2-external ID

[Jeremy Rosen]

> 
> -+BR2_EXTERNAL+ allows three different things:
> +A +BR2_EXTERNAL+ tree must contain at least those three files:
> +
> ++external.id+::
> +   That file should contain the ID for the +BR2_EXTERNAL+ tree.
> +   That ID is used to construct the +BR2_EXTERNAL_$(ID)+ variable,
> +   available in +Config.in+ and +external.mk+ (see below), so that ID
> +   must be a valid make and Kconfig variable. Buildroot sets
> +   +BR2_EXTERNAL_$(ID)+ to the path of the +BR2_EXTERNAL+ tree.
> ++
> +Examples:
> ++
> +  ** +FOO+ -> +BR2_EXTERNAL_FOO+
> +  ** +BAR_42+ -> +BR2_EXTERNAL_BAR_42+
> +  ** ...
> +

those are example of valid ID, but what is the exact syntax of the file ? 
is it a single line with just the ID ? this is not ver clear here....


Regards

Jeremy