system/skeleton: make nsswitch install conditional

Don't blindly install the /etc/nsswitch.conf file, it's useless for
toolchains that aren't (e)glibc-based and misleading.
Make the installation conditional on a (e)glibc toolchain.

[Thomas: use $(INSTALL) instead of cp.]

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>