core: generate all br2-external files in one go

core: generate all br2-external files in one go

When we introduced support for multiple br2-external trees, we
introduced two files, one on the Makefile side, needed very early,
and one on the kconfig side, needed later in the configuration
process. We naturally introduced a two-step generation, as it looked
like the simplest and most obvious way.

But now, we are on the verge of generating more files on the kconfig
side, and it does not make sense to add even more steps to generate
them.

And even better yet, we can generate both the Makefile-side and
kconfig-side files at the same time, in fact.

Make it so.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Vadim Kochan <vadim4j@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>