system: Fix warning when selecting systemd and ensure merged /usr workarounds are enabled

With systemd and a custom skeleton, we end up with the following warning:

warning: (BR2_INIT_SYSTEMD) selects BR2_ROOTFS_MERGED_USR which has
unmet direct dependencies (BR2_ROOTFS_SKELETON_DEFAULT)

Which makes sense as the logic to install the symlinks to /usr is part of
the default skeleton handling, but the BR2_ROOTFS_MERGED_USR symbol is ALSO
used by packages to activate various workarounds for a merged /usr, so it
should be always active when using systemd even with a custom rootfs
skeleton (that by definition must use a merged /usr as required by systemd).

So introduce a dummy symbol with the same name for the custom skeleton case,
similar to how we handle the BR2_PACKAGE_BUSYBOX_SHOW_OTHERS option.

This is a hidden option so it can only be activated by enabling systemd (and
not if people just want to use a merged /usr with a custom skeleton but not
systemd), but as this is really a legacy option that is probably good enough
(if not, patches are welcome!).

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>