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/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}"