ath79: enable factory.bin and adjust profile of ECB1750

ath79: enable factory.bin and adjust profile of ECB1750

factory.bin was not tested for ECB1750...
but it was tested on it's sister board ECB1200

The product ID for the header can be verified by inspecting
the header of OEM images, or in the u-boot environment.

Also:

  - the LAN LED is controlled directly by the AR8035 switch
  - the labelled (first increment) MAC for both is ethaddr (eth0)
  - list packages in alphabetical order
  - use default sysupgrade.bin recipe

Signed-off-by: Michael Pratt <mcpratt@pm.me>