Fix #5177 - set MRUBY_CONFIG early

Rakefile

@@ -14,7 +14,8 @@ require "mruby/core_ext"
 require "mruby/build"
 
 # load configuration file
-MRUBY_CONFIG = MRuby::Build.load_config
+MRUBY_CONFIG = MRuby::Build.mruby_config_path
+load MRUBY_CONFIG
 
 # load basic rules
 MRuby.each_target do |build|

lib/mruby/build.rb

@@ -39,7 +39,7 @@ module MRuby
   class Build
     class << self
       attr_accessor :current
-      def load_config
+      def mruby_config_path
         path = ENV['MRUBY_CONFIG'] || ENV['CONFIG']
         if path.nil? || path.empty?
           path = "#{MRUBY_ROOT}/build_config/default.rb"
@@ -47,7 +47,6 @@ module MRuby
           f = "#{MRUBY_ROOT}/build_config/#{path}.rb"
           path = File.exist?(f) ? f : File.extname(path).empty? ? f : path
         end
-        load path
         path
       end
     end