Check mrbgem.rake instead of directory to ignore empty directories in full-core.

mrbgems/full-core.gembox

@@ -2,8 +2,8 @@ MRuby::GemBox.new do |conf|
   conf.gem :core => "mruby-sprintf"
   conf.gem :core => "mruby-print"
 
-  Dir.glob("#{root}/mrbgems/mruby-*") do |x|
-    g = File.basename(x)
+  Dir.glob("#{root}/mrbgems/mruby-*/mrbgem.rake") do |x|
+    g = File.basename File.dirname x
     conf.gem :core => g unless g =~ /^mruby-(print|sprintf)$/
   end
 end