ukify: raise error if genkey is called with no output arguments

The idea is that genkey is called with either
--secureboot-private-key= + --secureboot-certificate=, and then it
writes those, or with --pcr-private-key + optionally --pcr-public-key
and then it writes those, or both. But when called with no arguments
whatsover, it did nothing.

There is no implicit value for any of those parameters as input (unlike in
mkosi), so we also don't want to have implicit values when used as output.
But we shouldn't return success if no work was done, this is quite confusing.

src/ukify/ukify.py

@@ -932,6 +932,8 @@ def generate_priv_pub_key_pair(keylength : int = 2048) -> tuple[bytes]:
 
 
 def generate_keys(opts):
+    work = False
+
     # This will generate keys and certificates and write them to the paths that
     # are specified as input paths.
     if opts.sb_key or opts.sb_cert:
@@ -947,6 +949,8 @@ def generate_keys(opts):
         print(f'Writing SecureBoot certificate to {opts.sb_cert}')
         opts.sb_cert.write_bytes(cert_pem)
 
+        work = True
+
     for priv_key, pub_key, _ in key_path_groups(opts):
         priv_key_pem, pub_key_pem = generate_priv_pub_key_pair()
 
@@ -957,6 +961,11 @@ def generate_keys(opts):
             print(f'Writing public key for PCR signing to {pub_key}')
             pub_key.write_bytes(pub_key_pem)
 
+        work = True
+
+    if not work:
+        raise ValueError('genkey: --secureboot-private-key=/--secureboot-certificate= or --pcr-private-key/--pcr-public-key must be specified')
+
 
 def inspect_section(opts, section):
     name = section.Name.rstrip(b"\x00").decode()