Bootstrap support for forky.

Until now we've been happily building all version × suite combinations
but that's not possible anymore (armel was dropped).

Generating an empty recipe is just a proof of concept, based on the idea
the Python script contains the logic, and the Makefile doesn't/shouldn't
know about those details.

generate-recipe.py

@@ -3,6 +3,7 @@
 import re
 import sys
 import subprocess
+from pathlib import Path
 
 # pylint: disable=invalid-name
 
@@ -18,12 +19,23 @@ if version not in ["1", "2", "3", "4"]:
     sys.exit(1)
 
 suite = sys.argv[2]
-if suite not in ['bullseye', 'bookworm', 'trixie']:
+if suite not in ['bullseye', 'bookworm', 'trixie', 'forky']:
     print("E: unsupported suite %s" % suite, file=sys.stderr)
     sys.exit(1)
 target_yaml = 'raspi_%s_%s.yaml' % (version, suite)
 
 
+### Detect unsupported combinations
+#
+# Some version/suite combinations are no longer supported (e.g. the architecture
+# got removed from the archive), in which case generate an empty file and let
+# the Makefile pick it up from there.
+if version == '1' and suite in ['forky']:
+    print(f'W: generating an empty recipe for unsupported combination ({version}×{suite})')
+    Path(target_yaml).write_text('')
+    sys.exit(0)
+
+
 ### Setting variables based on suite and version starts here
 
 # Arch, kernel, DTB: