change to config tool

[ron minnich]

The config tool has been changed to allow 'default' statements in the 
mainboard file.

Now, you can have 
default ROM_SIZE 512*1024

in the mainboard file, and over-ride that with (e.g.)
option ROM_SIZE=1024*1024

in the target file.

I've tested this and it works fine; it's now committed.

thanks

ron