busybox: handle syslog related files properly

busybox: handle syslog related files properly

If CONFIG_SYSLOGD is not enabled, then the related service file should
not be installed as it will always fail at system start-up. The error
message is as following.

	[FAILED] Failed to start System Logging Service.

The same logic applies to CONFIG_KLOGD.

So we should first check the configuration before we install these
service files.

[YOCTO #5302]

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>