Fix the cmdline.txt options to boot by label, not by partition

This is done in order to allow for booting from non-MMC media (i.e. USB thumbdrives)

raspi3.yaml

@@ -124,6 +124,12 @@ steps:
       apt-get clean
       rm -rf /var/lib/apt/lists
 
+  # Modify the kernel commandline we take from the firmware to boot from
+  # the partition labeled raspiroot instead of forcing it to mmcblk0p2
+  - chroot: root-fs
+    shell: |
+      sed -i 's/.dev.mmcblk0p2/LABEL=raspiroot/' ${ROOT?}/boot/firmware/cmdline.txt
+
   # TODO(https://github.com/larswirzenius/vmdb2/issues/24): remove once vmdb
   # clears /etc/resolv.conf on its own.
   - shell: |