The version of aufs4-standalone in both linux-yocto 4.9 (pyro) and yocto-kernel-cache (yocto-4.9) fail to build (to be honest, I've not tried the latter but the code is the same): CONFIG_AUFS_XATTR triggers failures due to missing *xattr inode_operations. Furthermore, the kernel's inode_operations (* rename) entry adds new parameter for the callback function is missing.
I have patches for this locally. Will queue them shortly.
root@qemux86-64:~# uname -a Linux qemux86-64 4.9.71-rt18-yocto-preempt-rt #1 SMP PREEMPT RT Sat Dec 23 23:46:40 EST 2017 x86_64 GNU/Linux root@qemux86-64:~# zcat /proc/config.gz | grep -i aufs CONFIG_AUFS_FS=y CONFIG_AUFS_BRANCH_MAX_127=y # CONFIG_AUFS_BRANCH_MAX_511 is not set # CONFIG_AUFS_BRANCH_MAX_1023 is not set # CONFIG_AUFS_BRANCH_MAX_32767 is not set CONFIG_AUFS_SBILIST=y # CONFIG_AUFS_HNOTIFY is not set # CONFIG_AUFS_EXPORT is not set # CONFIG_AUFS_XATTR is not set # CONFIG_AUFS_FHSM is not set # CONFIG_AUFS_RDU is not set # CONFIG_AUFS_DIRREN is not set # CONFIG_AUFS_SHWH is not set # CONFIG_AUFS_BR_RAMFS is not set # CONFIG_AUFS_BR_FUSE is not set CONFIG_AUFS_BDEV_LOOP=y # CONFIG_AUFS_DEBUG is not set root@qemux86-64:~#