[coreboot] [PATCH]various build system improvements

[Patrick Georgi]

Hi,

attached patch improves various aspects in the build system:

make only needs to read Makefile.incs once, thanks to the
SECONDEXPANSION feature of GNU make (and we rely on GNU make for lots of
things already)

File paths are relative to the root directory, which simplifies
debugging (make V=1 gives shorter command lines) and helps ccache
finding matches for checkouts in different directories (even though it
should normalize paths itself)

It's abuild tested.

Signed-off-by: Patrick Georgi