Building EMSCRIPTEN boards on the latest EMCC

[brendena]

So this pull is updated to work with the latest EMCC. So there's just two changes.

• One is a name change on the build
• The other is a javascript function definition.

I'm not entirely what changed to necessitate this function needing to be defined. But it seemed to fix the problem.

Error

error: undefined symbol: emscripten_memcpy_js (referenced by root reference (e.g. compiled C/C++ code))

[gfwilliams]

Great - thanks! Still seems to work with the old EMCC too