[coreboot] [PATCH]Improve ccache handling

[Patrick Georgi]

Hi,

attached patch improves ccache handling (and requires ccache 3.1 to work
properly, at least in my tests).

coreboot file names are now normalized to the base directory before
hashing, allowing multiple trees to share the same cache more
efficiently. This will greatly benefit the build bot.

It also fixes a bug ($(CCACHE) was never empty, so the test for the
existence of ccache never failed)

While we're at it, remove a redundant explicit use of ".config", to
allow to use config files of a different name slightly more easily in
the future.

The only thing that's accountable for cache misses on a seeded cache is
now the date/time values in config.h - I tested that by replacing them
with bogus values, but the runtime benefit was negligible.

I also found that we might be able to use __DATE__ and __TIME__
(preprocessor macros) instead of calling date(1). Any opinion on that?

Signed-off-by: Patrick Georgi