ath79-generic: TP-Link Archer C7 v2: Fix region selection for factory image (#3260)

The Archer C7 v2 needs a different factory image depending on the region
(EU or US); this got lost during the ar71xx -> ath79 migration.

The OpenWrt factory image without a region suffix is only installable on
very old TP-Link firmwares from before the region split was introduced.
The region-specific firmware works for both old and new TP-Link firmwares.

Fixes: 8fdda51f ("ath79-generic: (re)add support for archer-c7-v2 (#2437)")
(cherry picked from commit e721e7f7a6ba451b0de54a00ce622569326509da)

Co-authored-by: Matthias Schiffer <mschiffer@universe-factory.net>

targets/ath79-generic

@@ -411,6 +411,11 @@ device('teltonika-rut230-v1', 'teltonika_rut230-v1')
 
 -- TP-Link
 
+local tplink_region_suffix = ''
+if (env.GLUON_REGION or '') ~= '' then
+	tplink_region_suffix = '-' .. env.GLUON_REGION
+end
+
 device('tp-link-archer-a7-v5', 'tplink_archer-a7-v5', {
 	packages = ATH10K_PACKAGES_QCA9880,
 })
@@ -446,6 +451,7 @@ device('tp-link-archer-c6-v2-eu-ru-jp', 'tplink_archer-c6-v2', {
 
 device('tp-link-archer-c7-v2', 'tplink_archer-c7-v2', {
 	packages = ATH10K_PACKAGES_QCA9880,
+	factory = '-squashfs-factory' .. tplink_region_suffix,
 })
 
 device('tp-link-archer-c7-v4', 'tplink_archer-c7-v4', {