mvebu: Add turris-omnia.bootscript

mvebu: Add turris-omnia.bootscript

In contrast to the U-Boot version shipped with older versions of Turris
Omnia (CZ11NIC13, CZ11NIC20), the version shipped with Turris Omnia 2019
(CZ11NIC23) relies on the existence of /boot.scr.

Consequently, add a suitable boot script to the sysupgrade image.

Flash instructions for Turris Omnia 2019:
- Download openwrt-...-sysupgrade.img.gz, gunzip it, and copy the resulting
  .img file to the root of a USB flash drive (FAT32 or ext2/3/4).
- Enter a rescue shell: Either via 5-LED reset and ssh root@192.168.1.1
  on LAN port 4, or via 7-LED reset and the serial console.
- Insert the USB drive and mount it:
  mkdir /mnt; mount /dev/sda1 /mnt
- Flash the OpenWrt image to eMMC:
  dd if=/mnt/openwrt-...-sysupgrade.img of=/dev/mmcblk0 bs=4096 conv=fsync
- Reboot.

Flash instructions using a temporary "medkit" installation were written for
the older versions of Turris Omnia, and will *not* work on the Turris Omnia
2019.

Signed-off-by: Klaus Kudielka <klaus.kudielka@gmail.com>
Reviewed-by: Tomasz Maciej Nowak <tomek_n@o2.pl>
Tested-by: W. Michael Petullo <mike@flyn.org> (Turris Omnia "2020")