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>

rules.mk

@@ -298,6 +298,9 @@ ifneq ($(CONFIG_CCACHE),)
   TARGET_CXX:= ccache_cxx
   HOSTCC:= ccache $(HOSTCC)
   HOSTCXX:= ccache $(HOSTCXX)
+  export CCACHE_BASEDIR:=$(TOPDIR)
+  export CCACHE_DIR:=$(if $(call qstrip,$(CONFIG_CCACHE_DIR)),$(call qstrip,$(CONFIG_CCACHE_DIR)),$(TOPDIR)/.ccache)
+  export CCACHE_COMPILERCHECK:=%compiler% -dumpmachine; %compiler% -dumpversion
 endif
 
 TARGET_CONFIGURE_OPTS = \