Fixed sysupgrade.sh for TP-Link TL-WDR4900 v1

Signed-off-by: Steffen Pankratz <kratz00@gmx.de>
Reviewed-by: Tim Niemeyer <tim.niemeyer@mastersword.de>

bsp/default/root_file_system/etc/sysupgrade.sh

 #!/bin/sh
 
 BOARD=`uci get board.model.name`
-FILE="openwrt-ar71xx-generic-$BOARD-squashfs-sysupgrade.bin"
+
+case $BOARD in
+    tl-wdr4900-v1 )
+        SOC="mpc85xx" ;;
+    * )
+        SOC="ar71xx" ;;
+esac
+
+FILE="openwrt-${SOC}-generic-${BOARD}-squashfs-sysupgrade.bin"
 
 echo -ne "\nHardware: $BOARD\n"
 echo -ne "Downloading $FILE\n\n"