[package] base-files: remove rdate integration, add busybox ntpd init script and...

[package] base-files: remove rdate integration, add busybox ntpd init script and server list in /etc/config/system
The rdate applet proved to be too unreliable to obtain the current time on boot:
	- public time servers are rare and often unreachable or overloaded
	- rdate does not daemonize, it needs a network connection the moment it is started, leading to race conditions
	- the /etc/config/timeserver configuration is overly complex and there is no reliable way to disable rdate invocations
	- the time protocol as specified in RFC 868 is considered obsolete
This commit adds an init script /etc/init.d/sysntpd which starts and stops the busybox ntpd accordingly.
The builtin ntpd can be disabled by either disabling the init script, removing the symlink to busybox or
by clearing the timeserver list in /etc/config/system.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@28612 3c298f89-4303-0410-b956-a3cf2f4a3e73