[coreboot] Build system: Where do I add `-E` switch for GCC to view preprocessor output?

[Paul Menzel]

Dear coreboot folks,

I was unsuccessful in finding the correct place to add the `-E` switch
to `gcc` so I can see the preprocessor output.

Adding it to `CFLAGS` in `Makefile` and `Makefile.inc` resulted in a
non-working build. Changing `.xcompile` is not possible as it gets
rewritten when doing `make`.

Thanks,

Paul