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).

2 files changed, 22 insertions, 49 deletions

diff --git a/doc/guides/compile.md b/doc/guides/compile.md
index 233c3e594..ad57970e3 100644
--- a/doc/guides/compile.md
+++ b/doc/guides/compile.md
@@ -24,40 +24,29 @@ Optional:
 
 ## Build
 
-To compile `mruby`, just call `rake` inside of the mruby source
-root. To generate and execute the test tools call `rake test`. To
-clean all build files call `rake clean`. To see full command line on
-build, call `rake -v`.
-
-If you want to compile for the specific configuration, specify
-`MRUBY_TARGET` or `TARGET` environment variable, e.g
-
-```sh
-rake TAGRET=host
-```
-
-The default target is `host`.  The compilation target desciption files
-(with `.rb` suffix) are contained in the `target` directory.
-
-A build description file contains the build configuration of mruby and
-looks like the following for example:
-
+Inside of the root directory of the mruby source a file exists
+called *build_config.rb*. This file contains the build configuration
+of mruby and looks like this for example:
 ```ruby
 MRuby::Build.new do |conf|
   toolchain :gcc
 end
 ```
 
-All tools necessary to compile mruby can be set or modified here. In
-case you want to try different configuration, you can create a new
-configuration file under `target` and specify the configuration using
-the `MRUBY_TARGET` environment variable.
+All tools necessary to compile mruby can be set or modified here. In case
+you want to maintain an additional *build_config.rb* you can define a
+customized path using the *$MRUBY_CONFIG* environment variable. If the
+path doesn't exist, *build_config/${MRUBY_CONFIG}.rb* is used.
+
+To compile just call `rake` inside of the mruby source root. To
+generate and execute the test tools call `rake test`. To clean
+all build files call `rake clean`. To see full command line on
+build, call `rake -v`.
 
 ## Build Configuration
 
-To create a new configuration, copy the existing configuration in the
-`target` directory, and modify it. We wish you submit a pull-request,
-once you created a new configuration for a new platform.
+We wish you submit a pull-request to *build_config/**PLATFORM**.rb*, once you
+created a new configuration for a new platform.
 
 Inside of the configuration file, the following options can be
 configured based on your environment.
diff --git a/doc/mruby3.md b/doc/mruby3.md
index 029c9887e..e611a1b16 100644
--- a/doc/mruby3.md
+++ b/doc/mruby3.md
@@ -3,45 +3,29 @@ User visible changes in `mruby3`
 
 # Build System
 
-You can specify `TARGET` option to `rake` via a command line
-option, or via an environment variable, e.g.
-
-`rake TARGET=host all`
-
-or
-
-`TARGET=host rake all`
-
-It's much easier to switch multiple targets than the
-previous `build_config.rb` system.
-
 ## `presym` target
 
 The first compilation of `mruby` may require generation of a
 static symbol table named `build/presym`. You can generate
 the table by `rake gensym`.
 
-## `target` directory
+## `build_config` directory
 
-Build target specification files are loaded from `target`
-directory. The default `TARGET` is `host` which is described
-in `target/host.rb`. There are many other targets for example:
+Typical build configuration files are located in `build_config`
+directory. For examples:
 
 * `host-gprof`: compiles with `gprof` for performance tuning
 * `host-m32`: compiles in gcc 32bit mode on 64bit platforms
 * `boxing`: compiles all three boxing options
 * `clang-asan`: compiles with `clang`'s Address Sanitizer
 
-`target/host.rb` comes  with comments to help  writing a new
-target description.
-
-If you want to have your target description out of the
-source tree, you can specify the path to the description
-file in `MRUBY_CONFIG`.
+`build_config/${MRUBY_CONFIG}.rb` is used if the path specified
+in `MRUBY_CONFIG` doesn't exist, so you can specify it as
+`rake MRUBY_CONFIG=boxing`.
 
-# Build Target Contribution
+# Build Configuration Contribution
 
-When you write a new target description, please
+When you write a new build configuration description, please
 contribute. We welcome your contribution as a GitHub
 pull-request.