[Buildroot] [PATCH 3/3] infra: replace BUILDROOT_CONFIG with BR2_CONFIG.

[Jeremy Rosen]

> > 
> > which is adapted from the mkusers script... and it failed silently.
> 
>  Really silently? sed should complain on stderr that the file doesn't
> exist, but indeed the script will continue with an empty
> BR2_RASPAUDIO_PAACCESS variable.
> 

I missed the message in the build log, but indeed, sed did print an
 error with the usefull variable content on stderr

so this works as expected and is a case of "this is the best we can 
do" since we can't proactively check the scripts...