Move the console= parameter for the serial console.

See https://salsa.debian.org/raspi-team/image-specs/-/issues/57 for
detailed background.

Summary, e.g. on the Pi 4:
 - fresh build and first boot means:
    console=ttyS1,115200 console=tty0
 - after dpkg-reconfigure raspi-firmware has run, with the default
   settings:
    console=tty0 console=ttyS1,115200

Having some consistency across boots seems desirable (esp. when the Pi
fails to boot and the hints are on a serial console which might not be
wired), so insert the console= parameter for the serial console right
before the root= parameter.

Currently, the /etc/kernel/postinst.d/z50-raspi-firmware hook uses:

    ${pre_cmdline} root=$ROOTPART […]

and console= parameters are inserted via ${pre_cmdline}, so inserting
the serial console before root= should get us the same results.

raspi_master.yaml

@@ -137,14 +137,15 @@ steps:
       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
+  # the partition labeled raspiroot instead of forcing it to mmcblk0p2.
+  # Also insert the serial console right before the root= parameter.
   #
   # These changes will be overwritten after the hardware is probed
   # after dpkg reconfigures raspi-firmware (upon first boot), so make
   # sure we don't lose label-based booting.
   - chroot: /
     shell: |
-      sed -i 's/^/console=__SERIAL_CONSOLE__ /' /boot/firmware/cmdline.txt
+      sed -i 's/root=/console=__SERIAL_CONSOLE__ root=/' /boot/firmware/cmdline.txt
       sed -i 's#root=/dev/mmcblk0p2#root=LABEL=RASPIROOT#' /boot/firmware/cmdline.txt
       sed -i 's/^#ROOTPART=.*/ROOTPART=LABEL=RASPIROOT/' /etc/default/raspi-firmware