app-emulation/wa-linux-agent: Update Flatcar patch

When CoreosCommonUtil was factored out, we missed updating the class
name in a call to super(). This results in an error when executing
`/usr/sbin/waagent -force -deprovision+user`. Fix the class name.

Create a compatibility symlink at the old config file location
(/usr/share/oem/waagent.conf) to handle the case of enabling
auto-updates on the agent. The upstream version of the agent does not
have our downstream patch so doesn't know about the updated config file
location. We should upstream our changes.

Signed-off-by: Jeremi Piotrowski <jpiotrowski@microsoft.com>

sdk_container/src/third_party/coreos-overlay/app-emulation/wa-linux-agent/files/0001-flatcar-changes.patch

@@ -115,7 +115,7 @@ index 00000000..66eae16e
 +        # User 'core' is not a sysuser.
 +        if username == 'core':
 +            return False
-+        return super(CoreOSUtil, self).is_sys_user(username)
++        return super(CoreosCommonUtil, self).is_sys_user(username)
 +
 +    def is_dhcp_enabled(self):
 +        return True
@@ -404,7 +404,7 @@ new file mode 100644
 index 00000000..d0d6f7c8
 --- /dev/null
 +++ b/init/flatcar/waagent.service
-@@ -0,0 +1,30 @@
+@@ -0,0 +1,31 @@
 +[Unit]
 +Description=Microsoft Azure Linux Agent
 +Wants=network-online.target sshd.service sshd-keygen.service
@@ -419,6 +419,7 @@ index 00000000..d0d6f7c8
 +# /usr/share/flatcar/etc.
 +#
 +ExecStartPre=/bin/bash -c 'if [[ ! -e /etc/waagent.conf ]]; then ln -sf ../usr/share/waagent/waagent.conf /etc/waagent.conf; fi'
++ExecStartPre=/bin/bash -c 'if [[ ! -e /oem/waagent.conf ]]; then ln -sf /etc/waagent.conf /oem/waagent.conf; fi'
 +
 +# This could be done also with:
 +#