Makefile: Store OS release in /usr/lib/os-release

It is recommended that vendor trees store OS release information
in /usr/lib/os-release and that /etc/os-release should be a relative
symlink to /usr/lib/os-release.

For more details, see:

http://0pointer.de/public/systemd-man/os-release.html

[Peter: don't hide command, simplify ln invocation]
Signed-off-by: Chris Lesiak <chris.lesiak@licor.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

Makefile

@@ -729,7 +729,8 @@ endif
 		echo "ID=buildroot"; \
 		echo "VERSION_ID=$(BR2_VERSION)"; \
 		echo "PRETTY_NAME=\"Buildroot $(BR2_VERSION)\"" \
-	) >  $(TARGET_DIR)/etc/os-release
+	) >  $(TARGET_DIR)/usr/lib/os-release
+	ln -sf ../usr/lib/os-release $(TARGET_DIR)/etc
 
 	@$(call MESSAGE,"Sanitizing RPATH in target tree")
 	$(TOPDIR)/support/scripts/fix-rpath target