root.sh: Fix the imx6 boot script location

The boot script symlink was pointing to a file that got renamed.

Fixes Librem5/image-builder#19

root.sh

@@ -69,7 +69,7 @@ function setup_uboot()
     sed -e "s/##RFSPART##/1/" -i "${basedir}/boot/boot_sd.txt"
     mkimage -A arm -T script -O linux -d "${basedir}/boot/boot_sd.txt" "${basedir}/boot/boot_sd.scr"
     if [ "${pureos_board}" == "imx6" ]; then
-        ln -fs boot.scr "${basedir}/boot/6x_bootscript"
+        ln -fs boot_sd.scr "${basedir}/boot/6x_bootscript"
     else
         ln -fs "${basedir}/boot/boot_sd.scr" boot.scr
     fi