core/br2-external: fix reporting errors

core/br2-external: fix reporting errors

When a br2-external tree has an issue, e.g. a missing file, or does not
have a name, or the name uses invalid chars, we report that condition by
setting the variable BR2_EXTERNAL_ERROR.

That variable is defined in the script support/scripts/br2-external,
which outputs it on stdout, and checked by the Makefile.

Before d027cd75d09, stdout was explicitly redirected to the generated
.mk file, with   exec >"${ofile}"   as the Makefile and Kconfig
fragments were generated each with their own call to the script, and
the validation phase would emit the BR2_EXTERNAL_ERROR variable in the
Makefile fragment.

But with d027cd75d09, both the Makefile and Kconfig fragments were now
generated with a single call to the script, and as such the semantics of
the scripts changed, and only each of the actual generators, do_mk and
do_kconfig, had their out put redirected. Which left do_validate with
the default stdout. Which would emit BR2_EXTERNAL_ERROR on stdout.

In turn, the stdout of the script would be interpreted by as part of the
Makefile. But this does not end up very well when a br2-external tree
indeed has an error:

  - missing a external.desc file:

    Makefile:184: *** multiple target patterns.  Stop.

  - empty external.desc file:

    Config.in:22: can't open file "output/.br2-external.in.paths"

So we must redirect the output of the validation step to the
Makefile fragment, so that the error message is correctly caught by the
top-level Makefile.

Note that we don't need to append in do_mk, and we can do an overwrite
redirection: if we go so far as to call do_mk, it means there was no
error, and thus the fragment is empty.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Tested-by: Romain Naour <romain.naour@gmail.com>