Create conf/machine/raspi4-custom.md
# keep the original machine as an override with high prio
MACHINEOVERRIDES =. "raspberrypi4:"
require conf/machine/raspberrypi4.conf
MACHINE_PREFIX_raspi4-custom = "raspberrypi4"
# start of our costumizations:
RPI_USE_U_BOOT = "1"
ENABLE_UART = "1"
Modify the linux kernel with configuration fragments in Yocto
This also works for U-boot.