Set the blocksize for devices having only 4MB of flash for ramips
devices already using the new image build code.

Informations about the used flash chip are gathered from the OpenWrt
wiki, wikidevi, forums, OEM bootlogs or the compatible property in the
device tree source file.

The en25q32b from the AirLive Air3GII does not have 4k support in the
kernel.

For the following boards no information about the used flash chip could
be found and a 64k blocksize is assumed:

  - Ralink V11ST-FE
  - Ralink AP-RT3052-V22RW-2X2
  - MediaTek MT7628 EVB
  - MediaTek MT7621 EVB
  - UPVEL UR-326N4G
  - Buffalo WZR-AGL300NH

Signed-off-by: Mathias Kresin <dev@kresin.me>