raspi3-firmware overwrites u-boot in the beta server image

[CodeExecution]

I tried out the Ubuntu Server 18.04 ARM64 beta image on my Raspberry Pi 3 and when raspi3-firmware's script is run, the config.txt gets overwritten to use the Pi's built-in bootloader to boot the kernel and initramfs instead of u-boot.

There are multiple solutions to this:

*Stop using raspi3-firmware and start using u-boot.
*Stop using u-boot in the server image and start using raspi3-firmware and the Pi's built-in bootloader.
*Update raspi3-firmware so that if it detects a u-boot binary in /boot/firmware, it won't update the config.txt.
*Enable CONFIG_DMA_CMA=y in the generic ARM64 kernel and start using that with GRUB2 and u-boot instead.