Use `default' gembox by default.

I changed to use `full-core` gembox for debugging. It should have been
reverted before `mruby3` branch was merged.

build_config/default.rb

@@ -20,7 +20,7 @@ MRuby::Build.new do |conf|
   # conf.gem :git => 'git@github.com:mattn/mruby-onig-regexp.git', :branch => 'master', :options => '-v'
 
   # include the GEM box
-  conf.gembox 'full-core'
+  conf.gembox 'default'
 
   # C compiler settings
   # conf.cc do |cc|

build_config/host-cxx.rb

@@ -3,7 +3,7 @@ MRuby::Build.new do |conf|
 
   enable_debug
   # include the default GEMs
-  conf.gembox 'full-core'
+  conf.gembox 'default'
 
   # C compiler settings
   conf.cc.defines = %w(MRB_ENABLE_DEBUG_HOOK)

build_config/host-gprof.rb

@@ -3,7 +3,7 @@ MRuby::Build.new do |conf|
   toolchain :gcc
 
   # include the GEM box
-  conf.gembox 'full-core'
+  conf.gembox 'default'
 
   conf.cc.flags << '-pg'
   conf.linker.flags << '-pg'

build_config/host-m32.rb

@@ -3,7 +3,7 @@ MRuby::Build.new do |conf|
   toolchain :gcc
 
   # include the GEM box
-  conf.gembox 'full-core'
+  conf.gembox 'default'
 
   conf.cc.flags << '-m32'
   conf.linker.flags << '-m32'