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/functions/logging/logging.sh

@@ -227,6 +227,14 @@ function display_alert() {
 			inline_logs_color="${tool_color}" # either gray or normal, a bit subdued.
 			;;
 
+		ccache)
+			if [[ "${SHOW_CCACHE}" != "yes" ]]; then # ccache-related debugging messages, very very verbose
+				skip_screen=1
+			fi
+			level_indicator="🙈"
+			inline_logs_color="\e[1;34m" # blue; 36 would be cyan
+			;;
+
 		aggregation)
 			if [[ "${SHOW_AGGREGATION}" != "yes" ]]; then # aggregation (PACKAGE LISTS), very very verbose
 				skip_screen=1