[coreboot] [PATCH]improve support for parallel make in kconfig

[Patrick Georgi]

Hi,

attached patch helps with parallel make (make -j) on Kconfig builds on
my system.

Basically, $(obj)/build.h is made an explicit dependency of all the
various object files, so make doesn't try to build any of these with
build.h not around.

Also, build.h is atomically generated by writing to some other file,
then renaming.

I don't say that this "fixes" parallel make, as there might be more issues.

Signed-off-by: Patrick Georgi