Adjust support for CM3/CM4 devices.

See raspi-firmware's README.Debian for details but basically: both CM3
and CM4 in mainline have a DTB filename that's not expected by the
bootloader, so we need to tell the bootloader about the appropriate DTB
filename for those models specifically.

Do that via /etc/default/raspi-firmware-custom, adding a model filter
for some families:
 - [cm3] for raspi_3_* builds;
 - [cm4] for raspi_4_* builds.

Hint users of regular Pi devices into getting rid of those lines if they
notice them.

generate-recipe.py

@@ -122,6 +122,14 @@ if suite == 'buster':
 else:
     touch_machine_id = ''
 
+# Compute Module support:
+if version == '3':
+    cm_dev, cm_dtb = 'cm3', 'bcm2837-rpi-cm3-io3.dtb'
+elif version == '4':
+    cm_dev, cm_dtb = 'cm4', 'bcm2711-rpi-cm4-io.dtb'
+else:
+    cm_dev, cm_dtb = '', ''
+
 
 ### Write results:
 
@@ -157,7 +165,10 @@ with open('raspi_master.yaml', 'r') as in_file:
             .replace('__WIRELESS_FIRMWARE__', wireless_firmware) \
             .replace('__SERIAL_CONSOLE__', serial) \
             .replace('__HOST__', hostname) \
-            .replace('__TOUCH_MACHINE_ID__', touch_machine_id)
+            .replace('__TOUCH_MACHINE_ID__', touch_machine_id) \
+            .replace('__CM_DEV__', cm_dev) \
+            .replace('__CM_DTB__', cm_dtb) \
+            .replace('__VERSION__', version)
 
         out_text = align_replace(out_text, '__FIX_FIRMWARE_PKG_NAME__', fix_firmware_cmds)
         out_text = align_replace(out_text, '__EXTRA_ROOT_SHELL_CMDS__', extra_root_shell_cmds)