plugins: add support for disabling RTC and urandom plugins

Some systems may want to handle RTC and /dev/urandom by themselves, or
not at all as in the case of containers where the host does all this.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

configure.ac

@@ -81,6 +81,8 @@ AC_PLUGIN([resolvconf],    [no],  [Setup necessary files for resolvconf])
 AC_PLUGIN([x11-common],    [no],  [Console setup (for X)])
 AC_PLUGIN([netlink],       [yes], [Basic netlink plugin for IFUP/IFDN and GW events. Can be replaced with externally built plugin that links with libnl or similar.])
 AC_PLUGIN([hotplug],       [yes], [Setup and start udev or mdev hotplug daemon])
+AC_PLUGIN([rtc],           [yes], [Save and restore RTC using hwclock])
+AC_PLUGIN([urandom],       [yes], [Setup and save random seed at boot/shutdown])
 
 # Check for extra arguments or packages
 AC_ARG_WITH(heading,