gokrazy, Makefile: improve appliance build tooling

Several improvements to the gokrazy appliance build and flash workflow:

gokrazy/build.go:
  - Round Pi image size up to a power of 2 (QEMU raspi3b requires it)
  - Use monogok's mkfs.Perm for the /perm ext4 partition (pure Go,
    cross-platform, no e2fsprogs dependency)

gokrazy/mkfs:
  - Accept optional PermFile entries to include in the freshly-created
    ext4 filesystem (used for breakglass authorized_keys)
  - Use progresstracking.Ticker for flush progress reporting

gokrazy/tsapp*/config.json:
  - Point breakglass at /perm/breakglass.authorized_keys (not ec2)
  - Fix Pi SerialConsole to serial0,115200 (not ttyS0)

cmd/tailscale/cli/configure-flash-appliance.go:
  - Add --add-ssh-authorized-keys flag to write an authorized_keys
    file into /perm during flash (for breakglass SSH access)
  - Use progresstracking.CountingWriter + Ticker for write progress

Makefile:
  - tsapp-build-and-flash-pi: auto-include ~/.ssh/id_ed25519.pub
  - tsapp-qemu-pi: use virt machine + UEFI + ramfb + e1000 (working
    network + framebuffer), with DTB watchdog patch and
    gokrazy.log_to_serial for debugging
  - Auto-detect UEFI firmware path across Debian/Homebrew/Fedora

Updates #1866

Change-Id: Ifa97ad34c509a81e1637d9bce12a788037dfe5ec
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

gokrazy/build.go

@@ -30,7 +30,7 @@ var (
 	gaf    = flag.Bool("gaf", false, "if true, build a gokrazy archive format file instead of a full disk image")
 )
 
-// imageSizeBytes is the size of the disk image we ask monogok to
+// baseImageSizeBytes is the size of the disk image we ask monogok to
 // produce (and that the AWS AMI import expects). It has to be large
 // enough to fit gokrazy's standard partition layout (see
 // github.com/bradfitz/monogok/disklayout):
@@ -45,7 +45,25 @@ var (
 // file larger). The same value is passed to monogok via
 // --target_storage_bytes and to mkfs.Perm so the GPT and the ext4
 // inside it agree on the disk's size.
-const imageSizeBytes = 1258299392
+//
+// imageSizeBytesFor may round this up; callers should use that helper
+// instead of this constant.
+const baseImageSizeBytes = 1258299392
+
+// imageSizeBytesFor returns the disk image size to use for app. For Raspberry
+// Pi appliances the size is rounded up to the next power of two because
+// qemu-system-aarch64's raspi3b machine rejects SD card images whose size
+// isn't a power of two.
+func imageSizeBytesFor(app string) int64 {
+	if !strings.HasPrefix(app, "tsapp-pi.") {
+		return baseImageSizeBytes
+	}
+	n := int64(1)
+	for n < baseImageSizeBytes {
+		n <<= 1
+	}
+	return n
+}
 
 var conf gokrazyConfig
 
@@ -135,7 +153,7 @@ func buildImage() error {
 		args = append(args,
 			"overwrite",
 			"--full", filepath.Join(dir, *app+".img"),
-			fmt.Sprintf("--target_storage_bytes=%d", imageSizeBytes),
+			fmt.Sprintf("--target_storage_bytes=%d", imageSizeBytesFor(*app)),
 		)
 	}
 
@@ -156,7 +174,7 @@ func buildImage() error {
 		return fmt.Errorf("open %s: %w", imgPath, err)
 	}
 	defer f.Close()
-	if err := mkfs.Perm(f, imageSizeBytes); err != nil {
+	if err := mkfs.Perm(f, imageSizeBytesFor(*app)); err != nil {
 		return fmt.Errorf("formatting /perm in %s: %v", imgPath, err)
 	}
 	log.Printf("Wrote ext4 /perm filesystem to %s.", imgPath)