py: Allow to override port config file and thus have >1 configs per port.

Use it like:

make CFLAGS_EXTRA="-DMPCONFIG_FILE=mpconfigport_my.h"
> Bottom line: if you could refresh my memory on "" vs <> issue, I'd appreciate that, otherwise I'll try to look for it myself later.

git blame is a wonderful thing: found 8340c4838948876a4e477556beaed79e59e5e9b4 which is what you want.