ramips: support dual image feature on ALFA Network boards

New U-Boot version for MediaTek MT76x8/MT762x based ALFA Network boards
includes support for a 'dual image' feature. Users can enable it using
U-Boot environment variable 'dual_image' ('1' -> enabled).

When 'dual image' feature is enabled, U-Boot will modify DTB and divide
the original 'firmware' flash area into two, equal in size and aligned
to 64 KB partitions: 'firmware' and 'backup'. U-Boot will also adjust
size of 'firmware' area to match installed flash chip size.

U-Boot will load kernel from active partition which is marked with env
variable 'bootactive' ('1' -> first partition, '2' -> second partition)
and rename both partitions accordingly ('firmware' <-> 'backup').

There are 3 additional env variables used to control 'dual image' mode:
- bootlimit   - maximum number of unsuccessful boot tries (default: '3')
- bootcount   - current number of boot tries
- bootchanged - flag which informs that active partition was changed; if
                it is set and 'bootcount' reaches 'bootlimit' value,
                U-Boot will start web-based recovery which then updates
                both partitions with provided image

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>
(backported from commit bc173ddd83)

target/linux/ramips/base-files/etc/init.d/bootcount

@@ -5,8 +5,15 @@ START=99
 start() {
 	. /lib/functions.sh
 
-	local board=$(board_name)
-	if [ $board = "sk-wb8" ]; then
+	case $(board_name) in
+	alfa-network,ac1200rm|\
+	alfa-network,awusfree1|\
+	alfa-network,tube-e4g)
+		[ -n "$(fw_printenv bootcount bootchanged 2>/dev/null)" ] &&\
+			echo -e "bootcount\nbootchanged\n" | /usr/sbin/fw_setenv -s -
+		;;
+	sk-wb8)
 		fw_setenv bootcount 0
-	fi
+		;;
+	esac
 }