ccache.bbclass: Make it can be shared between different builds

CCACHE_BASEDIR: ccache removes this from file path, so that hashes will be
                the same in different build dirs.

CCACHE_TOP_DIR: Set it to a shared location for different builds.

(From OE-Core rev: 35d7fe73bba15de16d2eb0a4b12ef03b57b23306)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/ccache.bbclass

@@ -8,8 +8,28 @@
 #   Add the following line to the recipe if it can't be built with ccache:
 #   CCACHE_DISABLE = '1'
 #
+# - Share ccache files between different builds
+#   Set CCACHE_TOP_DIR to a shared dir
+#   CCACHE_TOP_DIR = /path/to/shared_ccache/
+#
+# - TO debug ccahe
+#   export CCACHE_DEBUG = "1"
+#   export CCACHE_LOGFILE = "${CCACHE_DIR}/logfile.log"
+#   And also set PARALLEL_MAKE = "-j 1" to get make the log in order
+#
 
-export CCACHE_DIR ?= "${TMPDIR}/ccache/${MULTIMACH_TARGET_SYS}/${PN}"
+# Set it to a shared location for different builds, so that cache files can
+# be shared between different builds.
+CCACHE_TOP_DIR ?= "${TMPDIR}/ccache"
+
+# ccahe removes CCACHE_BASEDIR from file path, so that hashes will be the same
+# in different builds.
+export CCACHE_BASEDIR ?= "${TMPDIR}"
+
+# Used for sharing cache files after compiler is rebuilt
+export CCACHE_COMPILERCHECK ?= "%compiler% -dumpspecs"
+
+export CCACHE_DIR ?= "${CCACHE_TOP_DIR}/${MULTIMACH_TARGET_SYS}/${PN}"
 
 # We need to stop ccache considering the current directory or the
 # debug-prefix-map target directory to be significant when calculating