yocto-kernel: add support for kernel feature add/rm/list

yocto-kernel: add support for kernel feature add/rm/list

Add yocto-kernel commands allowing users to add, remove, and list
kernel features with respect to a given BSP.

Features managed by these commands modify a special
machine-user-features.scc file associated with the kernel recipe
(.bbappend) of a yocto-bsp-generated BSP.  This is analagous to the
implementation of similar support for bare config items and patches
already implemented for yocto-bsp-generated BSPs.

Future patches will add support for providing a list of eligible
features as defined by linux-yocto kernels and locally-defined
(recipe-space) kernel features.

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>