kernel-yocto: Support in-tree config fragments

kernel-yocto: Support in-tree config fragments

[This is currently an RFC as it depends on a separate patch to add
  support for config fragments with .config file extensions in
  yocto-kernel-tools:
  https://lists.yoctoproject.org/g/linux-yocto/message/14431]

In Linux 6.8, the in-tree rv32_defconfig file for 32-bit RISC-V support
was dropped in favour of a Makefile target which combines the RISC-V
defconfig with a config fragment '32-bit.config'. So to build for 32-bit
RISC-V using the in-tree configuration from an upstream kernel, we need
to be able to enable this 32-bit config fragment.

We can support this via KERNEL_FEATURES if we add the in-tree arch
config directory (arch/${ARCH}/configs) to the search path.

While we're here, let's also add the generic config directory
(kernel/configs) to the search path so in-tree config fragments such as
'hardening.config' can be used.

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