ccache: Separate out into its own class

Currently, ccache is used if it is present. When building from scratch it gives
no performance improvement and creates a ton of empty directories even when its
not in use.

This change moves ccache support to a bbclass file which the user can choose to
enable. This should make builds more determinstic and make it easier/clearer
to the end user when its being used and when it is not.

(From OE-Core rev: 2acf8da4f13c175ea818b9514677b7059de1e3e2)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/base.bbclass

@@ -271,7 +271,7 @@ python base_eventhandler() {
 }
 
 addtask configure after do_patch
-do_configure[dirs] = "${CCACHE_DIR} ${S} ${B}"
+do_configure[dirs] = "${S} ${B}"
 do_configure[deptask] = "do_populate_sysroot"
 base_do_configure() {
 	:

meta/classes/ccache.bbclass

@@ -0,0 +1,5 @@
+CCACHE = "${@bb.which(d.getVar('PATH', True), 'ccache') and 'ccache '}"
+export CCACHE_DIR = "${TMPDIR}/ccache/${MULTIMACH_HOST_SYS}/${PN}"
+
+do_configure[dirs] =+ "${CCACHE_DIR}"
+do_kernel_configme[dirs] =+ "${CCACHE_DIR}"

meta/classes/kernel-yocto.bbclass

@@ -161,7 +161,7 @@ do_kernel_checkout[dirs] = "${S}"
 
 addtask kernel_checkout before do_patch after do_unpack
 
-do_kernel_configme[dirs] = "${CCACHE_DIR} ${S} ${B}"
+do_kernel_configme[dirs] = "${S} ${B}"
 do_kernel_configme() {
 	echo "[INFO] doing kernel configme"