Add platform-specific console configuration metadata

cosa gf-set-platform will read this, and create_disk.sh will save a JSON
version for the relevant architecture to /boot/coreos/platforms.json for
coreos-installer.

platforms.yaml

@@ -0,0 +1,62 @@
+# This file specifies image customizations that are specific to particular
+# architecture/platform pairs.  It is applied (indirectly, via
+# /boot/coreos/platforms.json) by gf-set-platform at build time and
+# coreos-installer at install time (if --platform is specified).  qemu and
+# metal don't use gf-set-platform and ignore this file.
+#
+# Currently this is used to configure the default console.  For any
+# arch/platform pairs not specified, GRUB and the kernel will apply their
+# own defaults.
+#
+# All architectures, platforms, and fields are optional.
+aarch64:
+  aws:
+    # https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/connect-to-serial-console.html#sc-considerations
+    grub_commands:
+      - serial --speed=115200
+      - terminal_input serial
+      - terminal_output serial
+    kernel_arguments:
+      - console=ttyS0,115200n8
+  packet:
+    # https://metal.equinix.com/developers/docs/resilience-recovery/serial-over-ssh/#limitations
+    grub_commands:
+      - serial --speed=115200
+      - terminal_input serial
+      - terminal_output serial
+    kernel_arguments:
+      - console=ttyAMA0,115200
+x86_64:
+  aws:
+    # https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/connect-to-serial-console.html#sc-considerations
+    grub_commands:
+      - serial --speed=115200
+      - terminal_input serial
+      - terminal_output serial
+    kernel_arguments:
+      - console=ttyS0,115200n8
+  azure:
+    # https://docs.microsoft.com/en-us/troubleshoot/azure/virtual-machines/serial-console-linux
+    grub_commands:
+      - serial --speed=115200
+      - terminal_input serial
+      - terminal_output serial
+    kernel_arguments:
+      - console=ttyS0,115200n8
+  gcp:
+    # Four serial ports are available; we use the first one
+    # https://cloud.google.com/compute/docs/troubleshooting/troubleshooting-using-serial-console
+    grub_commands:
+      - serial --speed=115200
+      - terminal_input serial
+      - terminal_output serial
+    kernel_arguments:
+      - console=ttyS0,115200n8
+  packet:
+    # https://metal.equinix.com/developers/docs/resilience-recovery/serial-over-ssh/#limitations
+    grub_commands:
+      - serial --unit=1 --speed=115200
+      - terminal_input serial
+      - terminal_output serial
+    kernel_arguments:
+      - console=ttyS1,115200n8