kernel-yocto: allow cfg fragments to be specified as required

kernel-yocto: allow cfg fragments to be specified as required

When .scc files are used, a configuration fragment can be declared
as "hardware" (required) or "non-hardware" (optional).

By default the configuration audit only warns about hardware
options. We can extend that default level of auditing to configuration
fragments that are on the SRC_URI or in KERNEL_FEATURES by
adding the ability to put a specifier after the fragment.

 i.e. KERNEL_FEATURES:append = 'hardening.config:required'

This is particular useful for fragments that are outside of
the kernel-cache (in a layer, in a kernel tree, etc)

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