kernel: Rearrange for 1.8

In 1.8 we want to streamline the kernel build process. Basically we
currently have multiple copies of the kernel source floating around
and the copying/compression/decompression is painful.

Lets assume we have a kernel source per machine since in most cases
this is true (and we have a sysroot per machine anyway). Basically,
instead of extracting a source into WORKDIR, then copying to a sysroot,
we now set S to point straight at STAGING_DIR_KERNEL.

Anything using kernel source can then just point at it and use:

do_configure[depends] += "virtual/kernel:do_patch"

to depend on the kernel source being present. Note this is different
behaviour to DEPENDS += "virtual/kernel" which equates to
do_configure[depends] += "virtual/kernel:do_populate_sysroot".

Once we do this, we no longer need the copy operation in
do_populate_sysroot, in fact there is nothing to do there (yay).

The remaining part of the challenge is to kill off the horrible
do_install. This patch splits it off to a different class, the idea here
is to have a separate recipe which depends on the virtual/kernel:do_patch
and just installs and packages the source needed to build modules on
target into a specific package.

Right now this code is proof of concept. It builds kernels and kernel
modules. perf blows up in do_package with issues on finding the kernel
version which can probably be fixed by adding back the right bit of do_install,
and adding a dependency of do_package[depends] += "virtual/kernel:do_install"
to perf. The whole thing needs a good write up, the corner cases testing
and probably a good dose of cleanup to the remaining code.

(From OE-Core rev: 3b3f7e785e27990ba21bc7cd97289c826a9a95d1)

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

meta/classes/kernel-yocto.bbclass

@@ -1,5 +1,3 @@
-S = "${WORKDIR}/linux"
-
 # remove tasks that modify the source tree in case externalsrc is inherited
 SRCTREECOVEREDTASKS += "do_kernel_link_vmlinux do_kernel_configme do_validate_branches do_kernel_configcheck do_kernel_checkout do_patch"