kernel-install: return 0 for unknown verbs in plugins

In practice this makes little difference, because kernel-install will
only call the plugins for 'add' or 'remove', and if we were to add a
new verb to kernel-install, we'd just change the plugins at the same
time. But our plugins serve as documentation for external plugins too,
and there it's better to silently ignore unknown verbs so that we can
add new verbs in the future.

(50-depomod.install was already like that.)

src/kernel-install/50-depmod.install

@@ -44,4 +44,5 @@ case "$COMMAND" in
         ;;
     *)
         exit 0
+        ;;
 esac

src/kernel-install/90-loaderentry.install

@@ -48,7 +48,7 @@ case "$COMMAND" in
     add)
         ;;
     *)
-        exit 1
+        exit 0
         ;;
 esac