Revert "system: Fix warning when selecting systemd"
This reverts commit b4718c3a28fc3b178253b24bd6fbb8540a724c7e.
The BR2_ROOTFS_MERGED_USR symbol is used to activate various workarounds for
a merged /usr by packages, so it should be active even with a custom rootfs
skeleton.
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>