kernel: move XFS_FS to extra kernel config, add additional platform with USER_NS support

With XFS_FS enabled USER_NS can't be enabled. We need user namespaces for example for chromium sandbox to work and for fully isolated linux containers.

I put XFS_FS to kernelExtraConfig until XFS_FS will not support user namespaces. I also added additional platform with enabled user namespaces.

If there is any nicer better way please let me know.