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"
Which unfortunately doesn't really say why it was reverted.

Fundamentally, the issue comes about when port A includes files from port B. Using "" says to look first in the directory that the including file (the .c file) is found in.

So if you have portA/mpconfigport.h and portB/somedriver.c (which has #include "mpconfigport.h") and portB/mpconfigport.h and portA's Makefile includes portB's somedriver.c, then it will always pull in portB's mpconfigport.h rather than portA's mpconfigport.h