Append per-board ccache statistics in log

Starting with ccache 4.4 it is possible to collect statistics about
cache miss/hit rates in a separate file.

Add the info of the build at end of created make.log file or on stdout.

Change-Id: I1bab712712f4d6379ec6733fdc55b234e3845da7
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/75087
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

toolchain.inc

@@ -10,6 +10,7 @@ endif
 
 export CCACHE_COMPILERCHECK=content
 export CCACHE_BASEDIR=$(top)
+export CCACHE_STATSLOG=$(obj)/ccache.stats
 
 $(foreach arch,$(ARCH_SUPPORTED), \
 	$(eval CC_$(arch):=$(CCACHE) $(CC_$(arch))))