U-boot with raspberry pi defaults to booting from the first FAT partition on the image.
To work well with a system update tool (RAUC) u-boot needs to somehow know to change to the updated partition when rebooting, how can this be made?

The image layout is as below:

mmcblk0      
|-mmcblk0p1  boot_A
|-mmcblk0p2  boot_B
|-mmcblk0p3  rootfs_A
|-mmcblk0p4  rootfs_B
`-mmcblk0p5  data

The simplest possible boot.scr script:

fdt addr ${fdt_addr} && fdt get value bootargs /chosen bootargs

fatload mmc 0:1 ${kernel_addr_r} @@KERNEL_IMAGETYPE@@
if test ! -e mmc 0:1 uboot.env; then saveenv; fi;
@@KERNEL_BOOTCMD@@ ${kernel_addr_r} - ${fdt_addr}

The output of fw_printenv is below:

BOOT_A_LEFT=3
BOOT_B_LEFT=3
BOOT_DEV=mmc 0:1
BOOT_ORDER=A B
arch=arm
baudrate=115200
board=rpi
board_name=Compute Module 4
board_rev=0x14
board_rev_scheme=1
board_revision=0xC03141
boot_a_script=load ${devtype} ${devnum}:${distro_bootpart} ${scriptaddr} ${prefix}${script}; source ${scriptaddr}
boot_efi_binary=load ${devtype} ${devnum}:${distro_bootpart} ${kernel_addr_r} efi/boot/bootaa64.efi; if fdt addr ${fdt_addr_r}; then bootefi ${kernel_addr_r} ${fdt_addr_r};else bootefi ${kernel_addr_r} ${fdtcontroladdr};fi
boot_efi_bootmgr=if fdt addr ${fdt_addr_r}; then bootefi bootmgr ${fdt_addr_r};else bootefi bootmgr;fi
boot_extlinux=sysboot ${devtype} ${devnum}:${distro_bootpart} any ${scriptaddr} ${prefix}${boot_syslinux_conf}
boot_net_usb_start=usb start
boot_pci_enum=pci enum
boot_prefixes=/ /boot/
boot_script_dhcp=boot.scr.uimg
boot_scripts=boot.scr.uimg boot.scr
boot_syslinux_conf=extlinux/extlinux.conf
boot_targets=mmc0 mmc1 mmc2 usb0 pxe dhcp 
bootargs=coherent_pool=1M 8250.nr_uarts=1 snd_bcm2835.enable_compat_alsa=0 snd_bcm2835.enable_hdmi=1  smsc95xx.macaddr=E4:5F:01:C7:56:B1 vc_mem.mem_base=0x3ec00000 vc_mem.mem_size=0x40000000  dwc_otg.lpm_enable=0 console=ttyS0,115200 root=/dev/mmcblk0p2 rootfstype=ext4 rootwait nohz=off systemd.unified_cgroup_hierarchy=0 cgroup_enable=memory cgroup_enable=cpuset cgroup_enable=devices root=/dev/mmcblk0p3 rauc.slot=A
bootcmd=run distro_bootcmd
bootcmd_dhcp=devtype=dhcp; run boot_net_usb_start; run boot_pci_enum; if dhcp ${scriptaddr} ${boot_script_dhcp}; then source ${scriptaddr}; fi;setenv efi_fdtfile ${fdtfile}; setenv efi_old_vci ${bootp_vci};setenv efi_old_arch ${bootp_arch};setenv bootp_vci PXEClient:Arch:00011:UNDI:003000;setenv bootp_arch 0xb;if dhcp ${kernel_addr_r}; then tftpboot ${fdt_addr_r} dtb/${efi_fdtfile};if fdt addr ${fdt_addr_r}; then bootefi ${kernel_addr_r} ${fdt_addr_r}; else bootefi ${kernel_addr_r} ${fdtcontroladdr};fi;fi;setenv bootp_vci ${efi_old_vci};setenv bootp_arch ${efi_old_arch};setenv efi_fdtfile;setenv efi_old_arch;setenv efi_old_vci;
bootcmd_mmc0=devnum=0; run mmc_boot
bootcmd_mmc1=devnum=1; run mmc_boot
bootcmd_mmc2=devnum=2; run mmc_boot
bootcmd_pxe=run boot_net_usb_start; run boot_pci_enum; dhcp; if pxe get; then pxe boot; fi
bootcmd_usb0=devnum=0; run usb_boot
bootdelay=2
bootfstype=fat
bootpart=/dev/mmcblk0p3
cpu=armv8
devplist=1 2
dhcpuboot=usb start; dhcp u-boot.uimg; bootm
distro_bootcmd=for target in ${boot_targets}; do run bootcmd_${target}; done
distro_bootpart=2
efi_dtb_prefixes=/ /dtb/ /dtb/current/
ethaddr=e4:5f:01:c7:56:b1
fdt_addr=2eff2900
fdt_addr_r=0x02600000
fdt_high=ffffffffffffffff
fdtaddr=2eff2900
fdtcontroladdr=3af42770
fdtfile=broadcom/bcm2711-rpi-cm4.dtb
fileaddr=80000
filesize=1559a00
initrd_high=ffffffffffffffff
kernel_addr_r=0x00080000
load_efi_dtb=load ${devtype} ${devnum}:${distro_bootpart} ${fdt_addr_r} ${prefix}${efi_fdtfile}
loadaddr=0x1000000
mmc_boot=if mmc dev ${devnum}; then devtype=mmc; run scan_dev_for_boot_part; fi
preboot=pci enum; usb start;
pxefile_addr_r=0x02500000
ramdisk_addr_r=0x02700000
raucslot=A
scan_dev_for_boot=echo Scanning ${devtype} ${devnum}:${distro_bootpart}...; for prefix in ${boot_prefixes}; do run scan_dev_for_extlinux; run scan_dev_for_scripts; done;run scan_dev_for_efi;
scan_dev_for_boot_part=part list ${devtype} ${devnum} -bootable devplist; env exists devplist || setenv devplist 1; for distro_bootpart in ${devplist}; do if fstype ${devtype} ${devnum}:${distro_bootpart} bootfstype; then run scan_dev_for_boot; fi; done; setenv devplist
scan_dev_for_efi=setenv efi_fdtfile ${fdtfile}; for prefix in ${efi_dtb_prefixes}; do if test -e ${devtype} ${devnum}:${distro_bootpart} ${prefix}${efi_fdtfile}; then run load_efi_dtb; fi;done;run boot_efi_bootmgr;if test -e ${devtype} ${devnum}:${distro_bootpart} efi/boot/bootaa64.efi; then echo Found EFI removable media binary efi/boot/bootaa64.efi; run boot_efi_binary; echo EFI LOAD FAILED: continuing...; fi; setenv efi_fdtfile
scan_dev_for_extlinux=if test -e ${devtype} ${devnum}:${distro_bootpart} ${prefix}${boot_syslinux_conf}; then echo Found ${prefix}${boot_syslinux_conf}; run boot_extlinux; echo SCRIPT FAILED: continuing...; fi
scan_dev_for_scripts=for script in ${boot_scripts}; do if test -e ${devtype} ${devnum}:${distro_bootpart} ${prefix}${script}; then echo Found U-Boot script ${prefix}${script}; run boot_a_script; echo SCRIPT FAILED: continuing...; fi; done
scriptaddr=0x02400000
serial#=1000000090ff6bb9
soc=bcm283x
stderr=serial,vidconsole
stdin=serial,usbkbd
stdout=serial,vidconsole
usb_boot=usb start; if usb dev ${devnum}; then devtype=usb; run scan_dev_for_boot_part; fi
usbethaddr=e4:5f:01:c7:56:b1
vendor=raspberrypi

The RPI always mounts /dev/mmcblk0p1 at /boot, which is not wanted, as every updated the mounted partition should change between /dev/mmcblk0p1 and /dev/mmcblk0p2. My suspicion is that u-boot always chooses the default boot action, which is bootcmd=run distro_bootcmd. This allows for changing the rootfs very neatly, but boot remains the same every time. Which u-boot variable indicates the booted partition?