Fix flannel support on Fedora CoreOS

* Fedora CoreOS now ships systemd-udev's `default.link` while
Flannel relies on being able to pick its own MAC address for
the `flannel.1` link for tunneled traffic to reach cni0 on
the destination side, without being dropped
* This change first appeared in FCOS testing-devel 32.20200624.20.1
and is the behavior going forward in FCOS since it was added
to align FCOS network naming / configs with the rest of Fedora
and address issues related to the default being missing
* Flatcar Linux (and Container Linux) has a specific flannel.link
configuration builtin, so it was not affected
* https://github.com/coreos/fedora-coreos-tracker/issues/574#issuecomment-665487296

Note: Typhoon's recommended and default CNI provider is Calico,
unless `networking` is set to flannel directly.

google-cloud/fedora-coreos/kubernetes/fcc/controller.yaml

@@ -182,6 +182,13 @@ storage:
         inline: |
           net.ipv4.conf.default.rp_filter=0
           net.ipv4.conf.*.rp_filter=0
+    - path: /etc/systemd/network/50-flannel.link
+      contents:
+        inline: |
+          [Match]
+          OriginalName=flannel*
+          [Link]
+          MACAddressPolicy=none
     - path: /etc/systemd/system.conf.d/accounting.conf
       contents:
         inline: |

google-cloud/fedora-coreos/kubernetes/workers/fcc/worker.yaml

@@ -109,6 +109,13 @@ storage:
         inline: |
           net.ipv4.conf.default.rp_filter=0
           net.ipv4.conf.*.rp_filter=0
+    - path: /etc/systemd/network/50-flannel.link
+      contents:
+        inline: |
+          [Match]
+          OriginalName=flannel*
+          [Link]
+          MACAddressPolicy=none
     - path: /etc/systemd/system.conf.d/accounting.conf
       contents:
         inline: |