ar71xx: improve support for TP-Link CPE510 v2

This fixes commit bae927c551 ("ar71xx: add support for TP-LINK CPE510
V2.0") where the support for this device wasn't optimal.

Device support for the CPE510v2 so far has been a hack to enable
flashing with CPE510v1 images. Those even have different hardware (e.g.
additional ethernet port).

With this patch, we provide proper support for this device in ar71xx.

Installation:
- Flash factory image through stock firmware WEB UI or through TFTP
- To get to TFTP recovery just hold reset button while powering on
  for around 4-5 seconds and release.
- Rename factory image to recovery.bin
- Stock TFTP server IP: 192.168.0.100
- Stock device TFTP address: 192.168.0.254

Fixes: bae927c551 ("ar71xx: add support for TP-LINK CPE510 V2.0")
Signed-off-by: Andrew Cameron <apcameron@softhome.net>
[Rebased onto revert commit, changed comments in mach-cpe510.c,
changed commit title and description, fixed eth0 MAC address,
removed eth1 initialization]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
[squashed revert, added fixes tag]
Signed-off-by: Petr Štetiar <ynezz@true.cz>

target/linux/ar71xx/base-files/lib/upgrade/platform.sh

@@ -586,6 +586,7 @@ platform_check_image() {
 		return $?
 		;;
 	cpe210|\
+	cpe510|\
 	eap120|\
 	wbs210|\
 	wbs510)
@@ -597,19 +598,9 @@ platform_check_image() {
 		tplink_pharos_check_image "$1" "01000000" "$(tplink_pharos_v2_get_model_string)" '\0\xff\r' && return 0
 		return 1
 		;;
-	cpe510)
-		local modelstr="$(tplink_pharos_v2_get_model_string)"
-		tplink_pharos_board_detect $modelstr
-		case $AR71XX_MODEL in
-		'TP-Link CPE510 v2.0')
-			tplink_pharos_check_image "$1" "7f454c46" "$modelstr" '\0\xff\r' && return 0
-			return 1
-			;;
-		*)
-			tplink_pharos_check_image "$1" "7f454c46" "$(tplink_pharos_get_model_string)" '' && return 0
-			return 1
-			;;
-		esac
+	cpe510-v2)
+		tplink_pharos_check_image "$1" "7f454c46" "$(tplink_pharos_v2_get_model_string)" '\0\xff\r' && return 0
+		return 1
 		;;
 	a40|\
 	a60|\