package/net-tools: use TARGET_CONFIGURE_OPTS to provide AR, CC and LDFLAGS

As noticed by Peter [1], use TARGET_CONFIGURE_OPTS to provide AR, CC and LDFLAGS.

Also remove TARGET_MAKE_ENV since it only contains PATH=$(BR_PATH) which is
also provided by TARGET_CONFIGURE_OPTS.

[1] http://lists.busybox.net/pipermail/buildroot/2016-May/162418.html

Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/net-tools/net-tools.mk

@@ -34,8 +34,7 @@ endef
 NET_TOOLS_POST_CONFIGURE_HOOKS += NET_TOOLS_ENABLE_I18N NET_TOOLS_ENABLE_IPV6
 
 define NET_TOOLS_BUILD_CMDS
-	$(TARGET_MAKE_ENV) AR="$(TARGET_AR)" CC="$(TARGET_CC)" \
-		LDFLAGS="$(TARGET_LDFLAGS)" \
+	$(TARGET_CONFIGURE_OPTS) \
 		LIBS="$(if $(BR2_NEEDS_GETTEXT_IF_LOCALE),-lintl)" \
 		$(MAKE) -C $(@D)
 endef