Switch fallback hostname to linuxkit-* from moby-*

This was missed when things were renamed.

The intention with this code was (apparently) to provide a (pseudo)unique
hostname in the case where something more specific was not provided (e.g. by
DHCP). Make this a little clearer by using '(none)' rather than 'linuxkit' as
the default, in the normal case this will be overwritten by something more
specific and if it isn't we will change it to something somewhat unique derived
from the MAC address (as before). nb: '(none)' is already used by Debian so I
think it is a safe choice as the sentinel value.

The use of both CONFIG_DEFAULT_HOSTNAME and the explicit /etc/hostname from
mkimage.sh is likely to be redundant in some cases, but neither seems to
completely cover all cases so keep both.

Signed-off-by: Ian Campbell <ian.campbell@docker.com>

examples/minimal.yml

@@ -2,7 +2,7 @@ kernel:
   image: "linuxkit/kernel:4.9.x"
   cmdline: "console=ttyS0 console=tty0 page_poison=1"
 init:
-  - linuxkit/init:42fe8cb1508b3afed39eb89821906e3cc7a70551
+  - linuxkit/init:1c8cd75ec89313f4058b069449e9bac966cd96b1
   - linuxkit/runc:b0fb122e10dbb7e4e45115177a61a3f8d68c19a9
   - linuxkit/containerd:60e2486a74c665ba4df57e561729aec20758daed
 onboot:

examples/node_exporter.yml

@@ -3,7 +3,7 @@ kernel:
   cmdline: "console=ttyS0 page_poison=1"
 init:
   - linuxkit/init:63eed9ca7a09d2ce4c0c5e7238ac005fa44f564b
-  - linuxkit/init:42fe8cb1508b3afed39eb89821906e3cc7a70551
+  - linuxkit/init:1c8cd75ec89313f4058b069449e9bac966cd96b1
   - linuxkit/runc:b0fb122e10dbb7e4e45115177a61a3f8d68c19a9
   - linuxkit/containerd:fe1b7f438a234cb6481c6538295115eac2a0596d
 services: