Use $config{build_file} instead of $target{build_file}

Use $config{build_file} instead of $target{build_file}

If the user specifies an alternative build file than the default, this
alternative is recorded in $config{build_file}, not $target{build_file}.
Therefore, the former should be used, leaving the latter as a mere default.

This is a bug.  While fixing it, document it better too.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/20173)