ccache.bbclass: Refactor it to make it more reliable

The previous ccache.bbclass has the following problems:
- It uses host's ccache for native recipes, but this may not work on some
  hosts, for example, it nerver works on my Ubuntu 14.04.4, there are always
  build failures (m4-native failed at do_configure, and others will also be
  failed if I disable CCACHE for m4-native)

- native/nativesdk/cross/crosssdk recipes use host's ccache, but target uses
  ccache-native, this may confuse user.

- The target recipes may use both host's ccache and ccache-native, this may
  cause unexpected problems and be hard to debug. This is because ccache-native is
  in SIGGEN_EXCLUDE_SAFE_RECIPE_DEPS, so ccache-native may not be present when
  rebuild target recipes, and then it would use hosttools/ccache, but the
  previous ccache files were generated by ccache-native.

- Target recipes can't use ccache when no ccache is installed on the host:
  CCACHE = "${@bb.utils.which(d.getVar('PATH'), 'ccache') and 'ccache '}"

After refactored:
All types recipes (native, target and others) will use ccache-native except
ccache-native itself, host's cache won't be used any more. It is more
reliable now, which will work everywhere when ccache-native can be built.

And now we need use "CCACHE_DISABLE = '1'" to disable ccache for the recipe
rather than "CCACHE = ''" since we set CCACHE in anonymous function, and
d.getVar('CCACHE') works after "CCACHE ??=" which is set in bitbake.conf, so we
can't check whether CCACHE is set or not in anonymous function since it is
always set. Use CCACHE_DISABLE to disable it would be more clear.

(From OE-Core rev: b25271b65262f70d849a4861da216c9be6c54d53)

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

meta/conf/bitbake.conf

@@ -492,7 +492,7 @@ HOSTTOOLS += " \
 HOSTTOOLS += "${@'ip ping ps scp ssh stty' if (bb.utils.contains_any('IMAGE_CLASSES', 'testimage testsdk', True, False, d) or any(x in (d.getVar("BBINCLUDED") or "") for x in ["testimage.bbclass", "testsdk.bbclass"])) else ''}"
 
 # Link to these if present
-HOSTTOOLS_NONFATAL += "aws ccache gcc-ar gpg ld.bfd ld.gold nc pigz sftp socat ssh sudo"
+HOSTTOOLS_NONFATAL += "aws gcc-ar gpg ld.bfd ld.gold nc pigz sftp socat ssh sudo"
 
 # Temporary add few more detected in bitbake world
 HOSTTOOLS_NONFATAL += "join nl size yes zcat"
@@ -504,10 +504,6 @@ HOSTTOOLS_NONFATAL += "bzr"
 HOSTTOOLS_NONFATAL += "scp"
 
 CCACHE ??= ""
-# ccache < 3.1.10 will create CCACHE_DIR on startup even if disabled, and
-# autogen sets HOME=/dev/null so in certain situations builds can fail.
-# Explicitly export CCACHE_DIR until we can assume ccache >3.1.10 on the host.
-export CCACHE_DIR ??= "${@os.getenv('HOME')}/.ccache"
 
 TOOLCHAIN_OPTIONS = " --sysroot=${STAGING_DIR_TARGET}"
 

meta/conf/layer.conf

@@ -49,7 +49,6 @@ SIGGEN_EXCLUDE_SAFE_RECIPE_DEPS += " \
   *->quilt-native \
   *->subversion-native \
   *->git-native \
-  *->ccache-native \
   *->icecc-create-env-native \
   gcc-cross-${TARGET_ARCH}->linux-libc-headers \
   ppp-dialin->ppp \