Makefile: fix rootfs overlay with BR2_ROOTFS_MERGED_USR enabled

Use rsync with '--keep-dirlinks' option to prevent rootfs overlay to
overwrite /usr, /bin, /sbin and /lib links in case BR2_ROOTFS_MERGED_USR
option is enabled.

Steps to reproduce failure:

- enable BR2_ROOTFS_MERGED_USR
- mkdir some_path/rootfs-overlay/lib/firmware/some_file.txt
- enable BR2_ROOTFS_OVERLAY="some_path/rootfs-overlay"
- run 'make'
- 'target/lib' contains only the files from 'some_path/rootfs-overlay/lib' instead
  of the original symlink 'lib -> usr/lib'.

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>