[coreboot] use gcc 4.6.0 link time optimization to reduce coreboot execution time

[Rudolf Marek]

> image size by removing unused functions and data. Reducing the
> image size saves boot time by minimizing the flash memory read

It always made me wonder how DMA support for flash memory copy could improve 
this. The SB700/SB800 has a support for that - it allows to copy the flash 
memory using DMA.

Thanks
Rudolf