Patch for V2 new config

[Mark Wilkinson]

Hi Ron,
    I started looking at building the Via/Epia with the v2, and noticed that your 
last snapshot said that the mainboard Config.lb should set the ROM_SIZE to 
265K.

Here's a little patch that lets the config python use 
'default OPTION value' tag in the Mainboard Config.lb overriding the value set 
in config/Optins.lb (which has ROM_SIZE set to NONE)

or should the command be 'default OPTION=<value>' in which case, alter the 
line for 'rule default<<C>>: ...' to read
    rule default<<C>>:  DEFAULT ID EQ value   {{ if(C): mbdefault(ID,value) }}

Regards
Mark Wilkinson.

PS. Hope to burn a V2 epia bios tomorrow morning and test !!