tool: Introduce b2s tool to help compute BLAKE2s digests

hw/application_fpga/tools/b2s/README.md

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+# b2s
+
+The firmware included a BLAKE2s digest of the expected device app in
+the first app slot. The firmware refuses to start the app if the
+computed digest differs from the constant.
+
+To simplify computing the digest, use this tool with the `-c` flag for
+including the digest in a C program:
+
+## Building
+
+`go build`
+
+## Running
+
+```
+./b2s -m b2s -c
+// BLAKE2s digest of b2s
+uint8_t digest[32] = {
+0x17, 0x36, 0xe9, 0x4e, 0xeb, 0x1b, 0xa2, 0x30, 0x89, 0xa9, 0xaa, 0xe, 0xf2, 0x6f, 0x35, 0xb2, 0xa9, 0x89, 0xac, 0x64, 0x63, 0xde, 0x38, 0x60, 0x47, 0x40, 0x91, 0x4e, 0xd7, 0x72, 0xa0, 0x58,
+};
+```
+
+To print the digest in a more user friendly way, leave out the `-c`:
+
+```
+./b2s -m b2s
+1736e94eeb1ba23089a9aa0ef26f35b2a989ac6463de38604740914ed772a058 b2s
+```

hw/application_fpga/tools/b2s/b2s.go

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+// SPDX-FileCopyrightText: 2025 Tillitis AB <tillitis.se>
+// SPDX-License-Identifier: BSD-2-Clause
+
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+
+	"golang.org/x/crypto/blake2s"
+)
+
+func usage() {
+	fmt.Printf("Usage: %s -m filename [-c]\n", os.Args[0])
+}
+
+func printCDigest(digest [blake2s.Size]byte, fileName string) {
+	fmt.Printf("// BLAKE2s digest of %v\n", fileName)
+	fmt.Printf("const uint8_t digest[32] = {\n")
+
+	for _, n := range digest {
+		fmt.Printf("0x%x, ", n)
+	}
+
+	fmt.Printf("\n}; \n")
+}
+
+func main() {
+	var messageFile string
+	var forC bool
+
+	flag.StringVar(&messageFile, "m", "", "Specify file containing message.")
+	flag.BoolVar(&forC, "c", false, "Print digest for inclusion in C program.")
+
+	flag.Usage = usage
+	flag.Parse()
+
+	if messageFile == "" {
+		usage()
+		os.Exit(0)
+	}
+
+	message, err := os.ReadFile(messageFile)
+	if err != nil {
+		fmt.Printf("%v\n", err)
+		os.Exit(1)
+	}
+
+	digest := blake2s.Sum256(message)
+
+	if forC {
+		printCDigest(digest, messageFile)
+	} else {
+		fmt.Printf("%x %s\n", digest, messageFile)
+	}
+
+	os.Exit(0)
+}

hw/application_fpga/tools/b2s/go.mod

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+module b2s
+
+go 1.23.0
+
+toolchain go1.23.7
+
+require golang.org/x/crypto v0.36.0
+
+require golang.org/x/sys v0.31.0 // indirect

hw/application_fpga/tools/b2s/go.sum

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+golang.org/x/crypto v0.36.0 h1:AnAEvhDddvBdpY+uR+MyHmuZzzNqXSe/GvuDeob5L34=
+golang.org/x/crypto v0.36.0/go.mod h1:Y4J0ReaxCR1IMaabaSMugxJES1EpwhBHhv2bDHklZvc=
+golang.org/x/sys v0.31.0 h1:ioabZlmFYtWhL+TRYpcnNlLwhyxaM9kWTDEmfnprqik=
+golang.org/x/sys v0.31.0/go.mod h1:BJP2sWEmIv4KK5OTEluFJCKSidICx8ciO85XgH3Ak8k=