LEGAL File Generator for Builds

Due to the reason that a mruby user will probably separate the build directory from the source I propose with this change to generate an additional LEGAL with every build for every target, which is specifying which packages are used and under which license they are. A mruby user can then deploy the binary or library directly together with this LEGAL file. Due to the reason that I'm not a lawyer this is of course only my non-professional understanding of the legal responsibilities of a user but I guess it is much better as the current state which generates a binary without a clear information what is contained inside of it.