nss-mdns: unbreak after e1eba3d9 (system/skeleton: make nsswitch install conditional)

Fixes http://autobuild.buildroot.net/results/481/4819cacd4b7015ef2bfaaf1b3fc9ba33f9194315/



After commit e1eba3d9, the skeleton no longer has a /etc/nsswitch.conf by
the time nss-mdns gets built, so the sed invocation fails.

Work around it by installing the glibc one early if not already available
in the skeleton.

If we want to do further modification of nsswitch.conf in the future (for
E.G.  samba), then we should probably introduce some hooks to
GLIBC_COPY_NSSWITCH_FILE instead.

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