targets/rp2040: Add bazel picotool support

Change-Id: I58c6263fabeda3e26f0e3b49e0fa7bc962dba40b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/214861
Commit-Queue: Auto-Submit <auto-submit@pigweed-service-accounts.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Erik Gilling <konkers@google.com>
Reviewed-by: Armando Montanez <amontanez@google.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>

WORKSPACE

@@ -519,6 +519,13 @@ git_repository(
     remote = "https://pigweed.googlesource.com/third_party/github/raspberrypi/pico-sdk",
 )
 
+# TODO: https://pwbug.dev/345244650 - Upstream bazel build.
+git_repository(
+    name = "picotool",
+    commit = "49072f6ebbc814dcc74d6f8b753b89c24af12971",
+    remote = "https://github.com/armandomontanez/picotool",
+)
+
 http_archive(
     name = "tinyusb",
     build_file = "@pico-sdk//src/rp2_common/tinyusb:tinyusb.BUILD",

targets/rp2040/py/BUILD.bazel

@@ -52,6 +52,7 @@ py_binary(
     ],
     data = [
         "//third_party/probe-rs",
+        "@picotool",
     ],
     main = "rp2040_utils/rpc_unit_test_runner.py",
     tags = ["manual"],

targets/rp2040/py/rp2040_utils/unit_test_runner.py

@@ -16,6 +16,7 @@
 
 import argparse
 import logging
+import os
 import subprocess
 import sys
 import time
@@ -42,7 +43,7 @@ try:
 
     r = runfiles.Create()
     _PROBE_RS_COMMAND = r.Rlocation('pigweed/third_party/probe-rs/probe-rs')
-    _PICOTOOL_COMMAND = 'picotool'
+    _PICOTOOL_COMMAND = r.Rlocation('picotool/picotool')
 except ImportError:
     _PROBE_RS_COMMAND = 'probe-rs'
     _PICOTOOL_COMMAND = 'picotool'
@@ -220,20 +221,39 @@ class PiPicoTestingDevice(SerialTestingDevice):
     def load_picotool_binary(self, binary: Path) -> bool:
         """Flash a binary to this device using picotool, returning success or
         failure."""
-        cmd = (
+        cmd = [
             _PICOTOOL_COMMAND,
             'load',
             '-x',
             str(binary),
+        ]
+
+        # If the binary has not file extension, assume that it is ELF and
+        # explicitly tell `picotool` that.
+        if not binary.suffix:
+            cmd += ['-t', 'elf']
+
+        cmd += [
             '--bus',
             str(self._board_info.bus),
             '--address',
             str(self._board_info.address()),
             '-F',
-        )
+        ]
+
         _LOG.debug('Flashing ==> %s', ' '.join(cmd))
+
+        # If the script is running inside Bazel, `picotool` needs to run from
+        # the project root so that it can find libusb.
+        cwd = None
+        if 'BUILD_WORKING_DIRECTORY' in os.environ:
+            cwd = os.environ['BUILD_WORKING_DIRECTORY']
+
         process = subprocess.run(
-            cmd, stdout=subprocess.PIPE, stderr=subprocess.STDOUT
+            cmd,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.STDOUT,
+            cwd=cwd,
         )
         if process.returncode:
             err = (