The way `--boot uefi` is implemented allows a value to be passed,
but the value is completely ignored. So `--boot uefi=off` or
`--boot uefi=FOOBAR` is treated the same as `--boot uefi`
Lets fix this by making `--boot uefi` and `--boot uefi=on` the same,
`--boot uefi=off` will be implemented later, but any other value
throws an error. This is technically an CLI break but I don't
think it's anything to worry about
Signed-off-by: Cole Robinson <crobinso@redhat.com>