userland: try to make userland executable selection saner

Only allow existing files to be built, stop extension expansion madness.

cli_function: get_cli print booleans properly, was printing without --no-
for negations.

build-userland-in-tree

@@ -22,7 +22,6 @@ https://github.com/cirosantilli/linux-kernel-module-cheat#userland-setup-getting
 ''',
             defaults={
                 'gcc_which': 'host',
-                'has_all_packages': True,
                 'in_tree': True,
                 'targets': ['.'],
             }