glibc: make nscd optional

NSCD itself is already packaged into its own package, but if it is never
going to be needed then it can be disabled and it won't be called from
glibc at all.  Add a PACKAGECONFIG for NSCD that is enabled by default.

As a side effect if NSCD is disable, glibc with and without systemd is
binary identical, which helps sstate reuse.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>