yocto-kernel: add support for creating recipe-space kernel features

yocto-kernel: add support for creating recipe-space kernel features

Add a yocto-kernel command allowing users to create a recipe-space
kernel feature local to a particular BSP.  The new feature is
subsequently available for the normal feature addition and removal
yocto-kernel commands used with features defined in the meta branch of
linux-yocto kernel repos.

(From meta-yocto rev: 13abcd93b9e1591bc45ff5f9eb17b8feb9ac9ae5)

Signed-off-by: Tom Zanussi <tom.zanussi@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>