Well, seL4 doesn't use U-boot's driver -- rather the ELFloader which loads the kernel has its own UART driver, and it uses that to print until it loads the kernel. From there, the kernel also has its own minimal UART driver, which it uses to print messages (if CONFIG_PRINTING is enabled).

Furthermore, after the kernel drops to userspace, userspace will simply avoid calling printf() until it's initialized a UART driver. After userspace has initialized its own UART driver, it will start calling printf().

So there are actually several UART drivers in play.

Kofi Doku Atuah
Kernel engineer
DATA61 | CSIRO