Implement serial-console for systemd.

Update install.d/20-stty so that we try to start
a getty on ttyS0 and ttyS1 when using systemd.

Change-Id: Id5b605c74bf82855af0af4d0f95659cdd3a7b3a0
Wait for tgtd socket to be available

tgtd returns execution control and backgrounds itself almost immediately
and before it has made it's listening socket available. This can cause a
race condition as the tgtd socket is not available when tgtadm is run,
resulting in an error:

failed to send request hdr to tgt daemon

Add a function to check if the socket is available before moving on to
calling tgtadm, and a wait_for helper function we can use.  We'll
check for the socket every 0.5 seconds, for up to 5 seconds.

I'm seeing this issue on almost every deploy using a ramdisk built from
Fedora 20. I'm not sure if something has changed in tgtd, but this
behavior is documented since Fedora 18 at least. In the systemd script
for tgtd, there is actually "sleep 5" to work around the problem.

See Also: https://bugzilla.redhat.com/show_bug.cgi?id=848942

Change-Id: Iffa9fc63393309ca653d592dff17316ecbea3e09
Merge "Implement serial-console for systemd."
Merge "Wait for tgtd socket to be available"