We have a dedicated headend bootloader on certain boards that will not print on the debug UART. Some boards have onboard ROMs that will print to debug UART on power on so it is not possible to completely silence the UART on those boards. Amlogic boards will print the search device during boot on the debug UART. Allwinner and Rockchip boards printing is controlled by the user software.
ATF
u-boot
# /boot/efi/boot.ini
silent=1
preboot=run bootcmd
Linux
# /etc/default/grub
GRUB_CMDLINE_LINUX_DEFAULT="console=ttynull"