bcm53xx: build images for Luxul ABR-4500 and XBR-4500 routers

Luxul ABR-4500 and XBR-4500 devices are wired routers with 5 Ethernet
ports and 1 USB 3.0 port. Flashing requires using Luxul firmware 6.4.0
or newer and uploading firmware using "Firmware Update" web UI page.

Signed-off-by: Dan Haab <dan.haab@legrand.com>