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"
Was that issue exactly with mpconfigport.h? The reason I changes it in this line is that if MPCONFIG_FILE is used, it will be "" too. And MPCONFIG_FILE case uses "" because  computed include requires macro to expand to "" or <> wrapped string, but passing such from make command line requires extensive quoting. Using stringify operator offered easy way to make "" string, so I went with that.

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.