util: tweak bls.options_append() support no/multiple options

The BLS specification [0] says the `options` field is optional and
can also appear multiple times. This commit tweaks the code to
deal with these corner cases and also adds tests that ensure that
this works correctly.

It also tweaks the file handling to be atomic.

[0] https://uapi-group.org/specifications/specs/boot_loader_specification/

osbuild/util/bls.py

@@ -3,6 +3,7 @@ Function for appending parameters to
 Boot Loader Specification (BLS).
 """
 import glob
+import os
 from typing import List
 
 
@@ -23,12 +24,16 @@ def options_append(root_path: str, kernel_arguments: List[str]) -> None:
     if len(bls_conf_files) == 0:
         raise RuntimeError(f"no BLS configuration found in {bls_glob}")
     for entry in bls_conf_files:
-        # Read in the file and then append to the options line.
         with open(entry, encoding="utf8") as f:
             lines = f.read().splitlines()
-        with open(entry, "w", encoding="utf8") as f:
+        with open(entry + ".tmp", "w", encoding="utf8") as f:
+            found_opts_line = False
             for line in lines:
-                if line.startswith('options '):
+                if not found_opts_line and line.startswith('options '):
                     f.write(f"{line} {' '.join(kernel_arguments)}\n")
+                    found_opts_line = True
                 else:
                     f.write(f"{line}\n")
+            if not found_opts_line:
+                f.write(f"options {' '.join(kernel_arguments)}\n")
+        os.rename(entry + ".tmp", entry)