build: improve ccache support

Set CCACHE_DIR to $(TOPDIR)/.ccache and CCACHE_BASEDIR to $(TOPDIR).
This allows to do clean and dirclean. Cache hit rate for test build
after dirclean is ~65%.
If CCACHE is enabled stats are printed out at the end of building process.
CCACHE_DIR config variable allows to override default, which could be useful
when sharing cache with many builds.
cacheclean make target allows to clean the cache.

Changes from v1:
- remove ccache directory using CCACHE_DIR variable
- remove ccache leftovers from sdk and toolchain make files
- introduce CONFIG_CCACHE_DIR variable
- introduce cacheclean make target

Signed-off-by: Roman Yeryomin <roman@advem.lv>

Makefile

@@ -62,6 +62,11 @@ dirclean: clean
 	rm -rf $(TMP_DIR)
 	$(MAKE) -C $(TOPDIR)/scripts/config clean
 
+cacheclean:
+ifneq ($(CONFIG_CCACHE),)
+	rm -rf $(if $(call qstrip,$(CONFIG_CCACHE_DIR)),$(call qstrip,$(CONFIG_CCACHE_DIR)),$(TOPDIR)/.ccache)
+endif
+
 ifndef DUMP_TARGET_DB
 $(BUILD_DIR)/.prepared: Makefile
 	@mkdir -p $$(dirname $@)
@@ -119,6 +124,9 @@ world: prepare $(target/stamp-compile) $(package/stamp-compile) $(package/stamp-
 	$(_SINGLE)$(SUBMAKE) -r package/index
 	$(_SINGLE)$(SUBMAKE) -r json_overview_image_info
 	$(_SINGLE)$(SUBMAKE) -r checksum
+ifneq ($(CONFIG_CCACHE),)
+	$(STAGING_DIR_HOST)/bin/ccache -s
+endif
 
 .PHONY: clean dirclean prereq prepare world package/symlinks package/symlinks-install package/symlinks-clean