armbian-next: better ccache logs/stats/Docker-support/use in u-boot/kernel

- introduce `SHOW_CCACHE=yes` for detailed ccache statistics and logging
- fix: kernel build is done under "env -i", so pass CCACHE_DIR down to Make if CCACHE_DIR is set...
- split from kernel.sh, show stats also for u-boot targets; show compile time
- add volume definition (under `${SRC}/cache/ccache`); auto-use that in Docker builds via `CCACHE_DIR`
- better logging for `do_with_ccache_statistics()`
- there's some CCACHE_DIR code from before; unify @TODO

lib/library-functions.sh

@@ -136,6 +136,15 @@ set -o errexit  ## set -e : exit the script if any statement returns a non-true
 # shellcheck source=lib/functions/compilation/atf.sh
 source "${SRC}"/lib/functions/compilation/atf.sh
 
+# no errors tolerated. invoked before each sourced file to make sure.
+#set -o pipefail  # trace ERR through pipes - will be enabled "soon"
+#set -o nounset   ## set -u : exit the script if you try to use an uninitialised variable - one day will be enabled
+set -o errtrace # trace ERR through - enabled
+set -o errexit  ## set -e : exit the script if any statement returns a non-true return value - enabled
+### lib/functions/compilation/ccache.sh
+# shellcheck source=lib/functions/compilation/ccache.sh
+source "${SRC}"/lib/functions/compilation/ccache.sh
+
 # no errors tolerated. invoked before each sourced file to make sure.
 #set -o pipefail  # trace ERR through pipes - will be enabled "soon"
 #set -o nounset   ## set -u : exit the script if you try to use an uninitialised variable - one day will be enabled