Revert `MRUBY_TARGET` mechanism; ref #5096

* 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).

1 files changed, 1 insertions, 13 deletions

diff --git a/README.md b/README.md
index 959abb0fc..5abf3445d 100644
--- a/README.md
+++ b/README.md
@@ -38,22 +38,10 @@ The URL of the mruby home-page is: https://mruby.org.
 
 We don't have a mailing list, but you can use [GitHub issues](https://github.com/mruby/mruby/issues).
 
-## How to compile and install (mruby and gems)
+## How to compile, test, and install (mruby and gems)
 
 See the [compile.md](https://github.com/mruby/mruby/blob/master/doc/guides/compile.md) file.
 
-## How to Build
-
-To build mruby, execute the following from the project's root directory.
-
-  $ rake TARGET=<target> test
-
-The default `TARGET` is `host`. `TARGET` will be loaded from `target` directory.
-
-Note: `bison` bundled with MacOS is too old to compile `mruby`.
-Try `brew install bison` and follow the instuction shown to update
-the `$PATH` to compile `mruby`.
-
 ## Building documentation
 
 There are two sets of documentation in mruby: the mruby API (generated by yard) and C API (Doxygen)