add escript support via plugin

- approach taken from https://github.com/synrc/mad/blob/master/src/mad_bundle.erl by @5HT 
- the escript generating code is on a single line as I couldn't find a simple  multi-line way of writing it that is still compatible with GNU Make 3.8.1
- I'm not sure if the dependency target rules are correct, e.g. `.PHONY` in build.config and core/core.mk, and in plugins/escript, core targets `escript:: distclean-escript all`
This emulator option defines the module used to run `main/1` from, if the script is ever renamed (e.g. so you know what release/git version it was built from), this ensures it won't mysteriously stop working. I'll remove `-smp auto` though.