coreos-base/oem-ec2-compat: use ttyS0 for console

On HVM instances, tty0 is actually a graphical interface. Since the
interface exposed is non-interactive, it's not possible to scroll
through the logs, which results in lots of missing information. Setting
the console to ttyS0, on the other hand, sends the output to the system
log.

PV instances don't have this issue since they are xen-based and use
hvc0.

sdk_container/src/third_party/coreos-overlay/coreos-base/oem-ec2-compat/files/grub-ec2.cfg

@@ -5,3 +5,10 @@ set oem_id="ec2"
 # Blacklist the Xen framebuffer module so it doesn't get loaded at boot
 # Disable `ens3` style names, so eth0 is used for both ixgbevf or xen.
 set linux_append="modprobe.blacklist=xen_fbfront net.ifnames=0"
+
+if [ "$grub_platform" = pc ]; then
+	set linux_console="console=ttyS0,115200n8"
+	serial com0 --speed=115200 --word=8 --parity=no
+	terminal_input serial_com0
+	terminal_output serial_com0
+fi