linux-yocto/3.4: allow kernel feature _appends to be overriden

linux-yocto/3.4: allow kernel feature _appends to be overriden

Updating the linux-yocto 3.4 recipe's feature flags to match the 3.8
recipe, which has the following change:

    It was pointed out that the current way the KERNEL_FEATURES variable
    is appended in the base linux-yocto recipe doesn't allow the appended
    features to be prevented in a layer without using python code and
    a recipe finalize hook.

    To allow easier overriding of 'extra' or 'optional' features that are
    defined in the linux-yocto recipe, we create a KERNEL_EXTRA_FEATURES
    variable. This variable can be set in a layer to define extra features
    or cleared to prevent the recipe's extra features from being appended
    to the core functionality.

(From OE-Core rev: 3bd592e832c6ce10947882f37564c12f4fa7f8e2)

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>