kernel-yocto: fix diffconfig/kernel_configme breakage

kernel-yocto: fix diffconfig/kernel_configme breakage

The bbclass did the following:

    do_diffconfig[depends] += "virtual/kernel:do_kernel_configme"

This clearly introduces a cross-kernel task dependency if the recipe
inheriting this class isn't the preferred provider of virtual/kernel, which is
obviously wrong, but further, will break the build if a kernel-yocto based
kernel is parsed and not skipped, but virtual/kernel refers to
a non-kernel-yocto recipe, which would not have the do_kernel_configme task.

Work around this by adding the in-recipe task dep programmatically with
bb.build.addtask when do_diffconfig exists.

(From OE-Core rev: 0e6881146d87f0d214d80bc92e54c113906db63a)

Signed-off-by: Christopher Larson <kergoth@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>