[coreboot] buildrom compiler settings patch

[Myles Watson]

This is a simple patch which consolidates the compiler settings.  I think it
might be better somewhere else, but at least it is consolidated.

Signed-off-by: Myles Watson

I'm also interested in why we don't set LD at the same time.  Is it an
oversight?

I'd like to remove the compiler settings in Build.settings and error
out if there isn't a CC defined by then.  Maybe CONFIG_CC, CONFIG_LD,
CONFIG_AS, etc. settings in the Build Settings menu of Kconfig makes
sense.

Thanks,
Myles