Commit 250bf6edd renamed KHASH_DEFAULT_SIZE to KHASH_INITIAL_SIZE but
missed updating build_config files and documentation. Also restore the
default value in khash.h to 32, consistent with the documented default
and the profile hierarchy (MRB_CONSTRAINED_BASELINE_PROFILE reduces it
to 16).
* In explanation of mruby, the expression `build_config.rb` is frequently
used including official documents, so I think that it will not make sense
if the file is no longer used.
* The `MRUBY_TARGET` mechanism seems to have little improvement, so I don't
think it should be changed to avoid unnecessary confusion.
* `MRUBY_TARGET` and `MRuby.targets` represent somewhat different things,
so using the same term "target" is a bit confusing.
The mechanism that can be written short when using a file under
`build_config` (renamed from `target`) directory remains
(`build_config/${MRUBY_CONFIG}.rb` is used if the path specified
in `MRUBY_CONFIG` doesn't exist).
HASUMI Hitoshi