gokrazy: add prototype Tailscale appliance, build tooling, docs

Updates #1866

Change-Id: I546316cb833bf2919e0d6f55cdc9951f375f165b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

gokrazy/tsapp/config.json

@@ -0,0 +1,24 @@
+{
+    "Hostname": "tsapp",
+    "Update": { "NoPassword": true },
+    "SerialConsole": "ttyS0,115200",
+    "Packages": [
+        "github.com/gokrazy/serial-busybox",
+        "github.com/gokrazy/breakglass",
+        "tailscale.com/cmd/tailscale",
+        "tailscale.com/cmd/tailscaled"
+    ],
+    "PackageConfig": {
+        "github.com/gokrazy/breakglass": {
+            "CommandLineFlags": [ "-authorized_keys=ec2" ]
+        },
+        "tailscale.com/cmd/tailscale": {
+            "ExtraFilePaths": {
+                "/usr": "usr-dir"
+            }
+        }
+    },
+    "KernelPackage": "github.com/tailscale/gokrazy-kernel",
+    "FirmwarePackage": "github.com/tailscale/gokrazy-kernel",
+    "InternalCompatibilityFlags": {}
+}