Add moby run packet to boot on baremetal Packet.net hosts

This uses the Packet.net API and iPXE to boot a Moby host.
There are several enhancements coming soon, such as SSH key
customisation, but this PR is sufficient to boot a host and
then use the web interface to get console access.

The user must currently upload the built artefacts to a public
URL and specify it via --base-url, e.g.:
moby run packet --api-key <key> --project-id <id> \
  --base-url http://recoil.org/~avsm/ipxe --hostname test-moby packet

See #1424 #1245 for related issues.

Signed-off-by: Anil Madhavapeddy <anil@docker.com>

examples/packet.yml

@@ -0,0 +1,50 @@
+kernel:
+  image: "mobylinux/kernel:4.9.x"
+  cmdline: "console=ttyS1 page_poison=1"
+init:
+  - mobylinux/init:e10e2efc1b78ef41d196175cbc07e069391f406e
+  - mobylinux/runc:b0fb122e10dbb7e4e45115177a61a3f8d68c19a9
+  - mobylinux/containerd:18eaf72f3f4f9a9f29ca1951f66df701f873060b
+  - mobylinux/ca-certificates:eabc5a6e59f05aa91529d80e9a595b85b046f935
+onboot:
+  - name: sysctl
+    image: "mobylinux/sysctl:2cf2f9d5b4d314ba1bfc22b2fe931924af666d8c"
+    net: host
+    pid: host
+    ipc: host
+    capabilities:
+     - CAP_SYS_ADMIN
+services:
+  - name: rngd
+    image: "mobylinux/rngd:3dad6dd43270fa632ac031e99d1947f20b22eec9@sha256:1c93c1db7196f6f71f8e300bc1d15f0376dd18e8891c8789d77c8ff19f3a9a92"
+    capabilities:
+     - CAP_SYS_ADMIN
+    oomScoreAdj: -800
+  - name: dhcpcd
+    image: "mobylinux/dhcpcd:0d4012269cb142972fed8542fbdc3ff5a7b695cd"
+    binds:
+     - /var:/var
+     - /tmp:/etc
+    capabilities:
+     - CAP_NET_ADMIN
+     - CAP_NET_BIND_SERVICE
+     - CAP_NET_RAW
+    net: host
+    oomScoreAdj: -800
+  - name: sshd
+    image: "mobylinux/sshd:160631d59fffc13d523ff7f09b3b49538d34b9cd"
+    capabilities:
+     - all
+    net: host
+    pid: host
+    binds:
+     - /root/.ssh:/root/.ssh
+     - /etc/resolv.conf:/etc/resolv.conf
+trust:
+  image:
+    - mobylinux/kernel
+files:
+  - path: root/.ssh/authorized_keys
+    contents: '#your ssh key here'
+outputs:
+  - format: kernel+initrd