ipq40xx: sysupgrade: allow flashing Linksys factory firmware

Allow forced flashing of a factory firmware image, after checking for the
correct FIT magic header and Linksys board-specific footer. Details of the
footer are already described in scripts/linksys-image.sh.

This is convenient as it avoids using a TFTP server or OEM GUI, and allows
restoring OEM firmware or installing a "breaking" OpenWrt update (e.g DSA
migration and kernel repartition) directly from the command line.

Devices supported at this time include EA6350v3, EA8300, MR8300 and WHW01.

Reviewed-by: Robert Marko <robimarko@gmail.com>
Tested-by:  Wyatt Martin <wawowl@gmail.com> # WHW01
Tested-by:  Tony Ambardar <itugrok@yahoo.com> # EA6350v3
Signed-off-by: Tony Ambardar <itugrok@yahoo.com>
(cherry picked from commit 6fc334cbdc)

target/linux/ipq40xx/base-files/lib/upgrade/linksys.sh

@@ -47,6 +47,14 @@ linksys_get_target_firmware() {
 	esac
 }
 
+linksys_is_factory_image() {
+	local board=$(board_name)
+	board=${board##*,}
+
+	# check matching footer signature
+	tail -c 256 $1 | grep -q -i "\.LINKSYS\.........${board}"
+}
+
 platform_do_upgrade_linksys() {
 	local magic_long="$(get_magic_long "$1")"
 
@@ -99,4 +107,14 @@ platform_do_upgrade_linksys() {
 		echo "writing \"$1\" image to \"$part_label\""
 		get_image "$1" | mtd write - "$part_label"
 	}
+
+	[ "$magic_long" = "d00dfeed" ] && {
+		if ! linksys_is_factory_image "$1"; then
+			echo "factory image doesn't match device"
+			return 1
+		fi
+
+		echo "writing \"$1\" factory image to \"$part_label\""
+		get_image "$1" | mtd -e "$part_label" write - "$part_label"
+	}
 }