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>

system/skeleton/etc/nsswitch.conf

@@ -1,17 +0,0 @@
-# /etc/nsswitch.conf
-#
-# Example configuration of GNU Name Service Switch functionality.
-# If you have the `glibc-doc-reference' and `info' packages installed, try:
-# `info libc "Name Service Switch"' for information about this file.
-
-passwd:         files
-group:          files
-shadow:         files
-hosts:          files dns
-networks:       files
-
-protocols:      files
-services:       files
-ethers:         files
-rpc:            files
-