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