ath79: image: disable sysupgrade images for routerstations and ja76pf2

ath79: image: disable sysupgrade images for routerstations and ja76pf2

Because a bug in handling partial erase blocks in 4.19 kernel, using
sysupgrade images will hard brick devices that use RedBoot bootloader
and have "FIS directory" with "RedBoot config" on the same erase block.
Since flashing the devices from bootloader is safe, and to not cause a
situation where external chip programmer or JTAG is needed, disable
sysupgrade images for affected boards while creating kernel.bin and
rootfs.bin for jjPlus JA76PF2 board, which doesn't have factory image.

To set up the JA76PF2 board follow "Installation" instructions in b3a0c97
("ath79: add support for jjPlus JA76PF2") except the part of loading
initramfs image and using sysupgrade image for flashing (point 6 and 7).
Enter following commands to flash the board from bootloader:
  fis init
  load -r -b 0x80060000 <openwrt_kernel_image_name>
  fis create linux
  load -r -b %{FREEMEMLO} <openwrt_rootfs_image_name>
  fis create rootfs
  fis load -l linux
  exec -c ""

For RouterStations use TFTP recovery procedure.

Ref: FS#2428
Cc: Matt Merhar <mattmerhar@protonmail.com>
Signed-off-by: Tomasz Maciej Nowak <tomek_n@o2.pl>