kernel.bbclass: Copy include/config/auto.conf in STAGING_KERNEL_BUILDDIR

kernel.bbclass: Copy include/config/auto.conf in STAGING_KERNEL_BUILDDIR

Linux commit aaed5c7739be ("kbuild: slim down package for building
external modules")[0] states that include/config/auto.conf is also a
file needed for out-of-tree build.

This avoids this error when building an out-of-tree Rust kernel module:
| make -C .../tmp/work-shared/qemux86-64/kernel-source M=$PWD
| make[1]: Entering directory '.../tmp/work-shared/qemux86-64/kernel-source'
| make[2]: Entering directory '.../tmp/work/qemux86_64-poky-linux/rust-out-of-tree-module/git/sources/rust-out-of-tree-module-git'
| .../tmp/work-shared/qemux86-64/kernel-source/Makefile:779: .../tmp/work-shared/qemux86-64/kernel-build-artifacts/include/config/auto.conf: No such file or directory

[0]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=aaed5c7739be81ebdd6008aedc8befd98c88e67a

Signed-off-by: Yoann Congal <yoann.congal@smile.fr>
Signed-off-by: Harish Sadineni <Harish.Sadineni@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>