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).

lib/mruby/build.rb

@@ -39,6 +39,17 @@ module MRuby
   class Build
     class << self
       attr_accessor :current
+      def load_config
+        path = ENV['MRUBY_CONFIG']
+        if path.nil? || path.empty?
+          path = "#{MRUBY_ROOT}/build_config.rb"
+        elsif !File.file?(path) && !Pathname.new(path).absolute?
+          f = "#{MRUBY_ROOT}/build_config/#{path}.rb"
+          path = File.exist?(f) ? f : File.extname(path).empty? ? f : path
+        end
+        load path
+        path
+      end
     end
     include Rake::DSL
     include LoadGems

lib/mruby/lockfile.rb

@@ -26,7 +26,7 @@ module MRuby
       end
 
       def instance
-        @instance ||= new("#{MRUBY_ROOT}/build/#{MRUBY_TARGET}.lock")
+        @instance ||= new("#{MRUBY_CONFIG}.lock")
       end
     end