emscript command line needs ability to set BoardKind

[gcohler]

Currently, there is a switch to set Setup -- but not BoardKind.

[biosbob]

i'll add a more general -D switch that allows any property to be defined at the command-line.... the board is simply 'em.lang.BoardKind=....' this allows us (say) to change the 'em.build.Optimize' property; or override some path property; etc....