Implement gembox-relative gemdir paths

In the case where a relative path is specified to a gembox from
build_config.rb, it was previously tricky to specify relative gem paths
from inside that gembox.

For example, consider a project in which mruby is checked out as a
submodule in the project root:

	+- project_root
	   |
	   +- mruby/
	   |  |
	   |  +- build_config.rb
	   |  |
	   |  +- ...
	   |
	   +- my_gembox/
	      |
	      +- my_gembox.gembox
	      |
	      +- my_gem/
	         |
	         +- mrbgem.rake
	         |
	         +- ...

If build_config.rb refers to my_gembox with a relative path, it's
difficult for my_gembox to then refer to my_gem. With this proposed
change, my_gembox.gembox can look like this:

	MRuby::GemBox.new do |conf|
	  conf.gem "my_gem"
	end

tasks/mrbgem_spec.rake

@@ -297,6 +297,8 @@ module MRuby
 
   GemBox = Object.new
   class << GemBox
+    attr_accessor :path
+
     def new(&block); block.call(self); end
     def config=(obj); @config = obj; end
     def gem(gemdir, &block); @config.gem(gemdir, &block); end

tasks/mruby_build_gem.rake

@@ -3,7 +3,10 @@ module MRuby
     def gembox(gemboxfile)
       gembox = File.expand_path("#{gemboxfile}.gembox", "#{MRUBY_ROOT}/mrbgems")
       fail "Can't find gembox '#{gembox}'" unless File.exists?(gembox)
+
       GemBox.config = self
+      GemBox.path = gembox
+
       instance_eval File.read(gembox)
     end
 
@@ -11,6 +14,8 @@ module MRuby
       caller_dir = File.expand_path(File.dirname(/^(.*?):\d/.match(caller.first).to_a[1]))
       if gemdir.is_a?(Hash)
         gemdir = load_special_path_gem(gemdir)
+      elsif gemdir.is_a?(String)
+        gemdir = "#{File.dirname(GemBox.path)}/#{gemdir}"
       else
         gemdir = File.expand_path(gemdir, caller_dir)
       end