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)
(cherry picked from commit
aa2d7e0ee15d1b7015479c38f370a25ceec690fc)