ukify: typo in doc and print when package is missing

If a package is missing, a subprocess is started with None as
command argument. Error raised by subprocess is therefore not helpful
at all to understand what needs to be done to fix that error.

Also fix doc since systemd-stub will look for .cmdline files, and not
.cmdline.efi files.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>

man/ukify.xml

@@ -435,7 +435,7 @@ Phases=enter-initrd:leave-initrd
       --secureboot-private-key=sb.key \
       --secureboot-certificate=sb.cert \
       --cmdline='debug' \
-      --output=debug.cmdline.efi
+      --output=debug.cmdline
       </programlisting>
 
       <para>This creates a signed PE binary that contains the additional kernel command line parameter

src/ukify/ukify.py

@@ -353,8 +353,10 @@ def find_tool(name, fallback=None, opts=None):
     if shutil.which(name) is not None:
         return name
 
-    return fallback
+    if fallback is None:
+        print(f"Tool {name} not installed!")
 
+    return fallback
 
 def combine_signatures(pcrsigs):
     combined = collections.defaultdict(list)