net-eth: added plus-shortcut

"use/stage2/net-eth use/net-eth" would be common enough
for installers or livecd images to just get these pulled in
together by a convenient shortcut thus reducing confusion
and chances to just forget one of these counterparts.

features.in/net-eth/README

@@ -5,3 +5,6 @@ udev-rule-generator-net штатно добавляется для обеспе
 сломали в апстримном systemd-udevd; для генерируемых "на лету"
 следует применять конфигурирование времени загрузки (NM, connman
 либо пакет livecd-net-eth).
+
+Обратите внимание: применение в инсталяторах требует задействования
+как use/net-eth, так и use/stage2/net-eth; +net-eth "подберёт" оба.

features.in/net-eth/config.mk

@@ -1,11 +1,13 @@
+# NB: this is aimed at LiveCD/installer images
++net-eth: use/net-eth use/stage2/net-eth
+
+# see also use/stage2/net-eth; do not depend on it though
+# as stage2-less images need preconfigured networking too
 use/net-eth: use/net
 	@$(call add_feature)
 	@$(call xport,NET_ETH)
 	@$(call add,BASE_PACKAGES,udev-rule-generator-net)
 
-# see also use/stage2/net-eth; do not depend on it though
-# as stage2-less images need preconfigured networking too
-
 # typical boilerplate
 use/net-eth/dhcp: use/net-eth use/net/dhcp
 	@$(call add,NET_ETH,eth0:dhcp)