U-Boot will not boot without the primary UART console enabled; the Pi
just hangs at the rainbow splash screen. There are three ways to enable
a UART console, but disabling Bluetooth is usually the best, as the
others have performance implications. Applications that need Bluetooth
will have to choose one of the other options, depending on their
specific requirements.
At this point, we can generate an SD card image that is composed of an
EFI system partition, a SquashFS root partition (a), a blank root
partition (b), and a data partition. On the EFI system partition are
the Raspberry Pi firmware and device trees, U-Boot and GRUB. The
SquashFS filesystem contains the kernel and the whole filesystem tree.
A Raspberry Pi can successfully boot from an SD card containing this
image. It's not quite usable yet, because there's no writable storage
available, or indeed any way to log in.
Dustin