Add an empty line before __EXTRA_CHROOT_SHELL_CMDS__

This means the generated recipes are getting two empty lines if there
are no such commands (that's the case for everyone right now), but this
emphasizes the existence of this placeholder, the same way as for its
__EXTRA_ROOT_SHELL_CMDS__ twin.

raspi_master.yaml

@@ -142,6 +142,7 @@ steps:
     shell: |
       sed -i 's/^/console=__SERIAL_CONSOLE__ /' /boot/firmware/cmdline.txt
       sed -i 's/.dev.mmcblk0p2/LABEL=RASPIROOT/' /boot/firmware/cmdline.txt
+
       __EXTRA_CHROOT_SHELL_CMDS__
 
   # TODO(https://github.com/larswirzenius/vmdb2/issues/24): remove once vmdb