hostname-setup: gracefully handle kernel with empty CONFIG_DEFAULT_HOSTNAME

Previously, sethostname_idempotent_full() calls gethostname_full() with
GET_HOSTNAME_ALLOW_NONE and GET_HOSTNAME_ALLOW_LOCALHOST flags. That
intended to get any values set by kernel. But, that does not work, as
the hostname may be empty.

Let's simplify the logic. The function sethostname_idempotent_full()
intends to set the requested hostname only when the current hostname
is different from the requested one. So, no check in getostname_full()
is required. Hence, simply use the result of uname() here.

Fixes #21896.