Add macros to <command> field in es_systems.cfg

[eib]

Adds %ES_HOME% and %ROM_DIR% macros to <command> fields in es_systems.cfg.

Fixes #680.

[tomaz82]

Not sure what this is supposed to fix?

You can have a portable setup in Windows using a batch file ( that's how I have done it the past 2 years )

[eib]

The idea is that you don't need an additional batch file.

I'm curious, though -- does your <command> use an absolute path to the batch file? Or does it depend on an environment variable? (I'd prefer to not use either of those approaches.)