[coreboot] build optimization

[Ronald Hoogenboom]

Hi,

I've attached a patch that removes the 3-mile-long compiler
commandlines, which vim's quickfix doesn't like so much. Instead of
putting all those -DXYZ='bla' on the compiler commandline, they are put
in a file called settings.h (as #define XYZ bla) and only a
--include=settings.h is put on the commandline.
This file is created unconditionally at the same time as when the
CPUFLAGS simply expanded make variable used to be created (not via a
target rule and dependency), so it shouldn't change anything.

Signed-off-by: Ronald Hoogenboom