Make note about MX_BUILD_EXPLODED conditional on it not being true

[smarr]

This is really a very minor things, though, every time I do a mx eclipseinit and it reminds me of MX_BUILD_EXPLODED, I spend a few minutes figuring out whether I got the config wrong.

By adding the check, the note is only printed when the configuration is something other than MX_BUILD_EXPLODED=true.

[smarr]

Hm, actually, I can't make anything work with MX_BUILD_EXPLODED=true set. Neither my own language nor GraalPy. All break with different issues. Some Java module issue or some dynamic library issue on GraalPy.