release: chase ports r519089 (rpi-firmware update)

The recent rpi-firmware update renamed "0" to "zero" in the RPi0 DTB
filename

It also included the components needed to boot the RPi4, so install those
now -- interested parties can install sysutils/u-boot-rpi4 and copy
config_rpi4.txt to config.txt on the FAT partition in order to boot the
board. Do note that we currently don't support ethernet/usb/pci.

Reviewed by: manu
MFC after: 3 days