lava/devices: rpi64: Use altbootcmd when needed

When running the aum-rollback-blacklist test, u-boot must rollback to
the older version when the bootlimit of 3 has been reached.

Using just `boot` will bypass the boot count check, so we need to add
the check here in the device template, like it is done for the other
device types.

Fixes: infrastructure/apertis-issues#129
Fixes: infrastructure/apertis-issues#130

Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>

lava/devices.yaml

@@ -39,7 +39,7 @@ device-arm64-bcm2711-rpi-4-b:
       board: rpi64
       boot_method: u-boot
       boot_commands:
-        - boot
+        - if test ${bootcount} -lt ${bootlimit}; then run distro_bootcmd; else run altbootcmd; fi
       deploy_device: sd-card
       device_type: bcm2711-rpi-4-b
       firststagedtb: broadcom/bcm2711-rpi-4-b.dtb