linux-yocto/3.8: allow kernel feature _appends to be overriden

linux-yocto/3.8: allow kernel feature _appends to be overriden

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: 779cb6e51fab87e00784bd97ec4771e69a79cf82)

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