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>

config/Config-devel.in

@@ -69,6 +69,13 @@ menuconfig DEVEL
 		help
 		  Compiler cache; see https://ccache.samba.org/
 
+	config CCACHE_DIR
+		string "Set ccache directory" if CCACHE
+		default ""
+		help
+		  Store ccache in this directory.
+		  If not set, uses './.ccache'
+
 	config EXTERNAL_KERNEL_TREE
 		string "Use external kernel tree" if DEVEL
 		default ""