buildman: Allow skipping the dtc build

For most boards, the device-tree compiler is built in-tree, ignoring the
system version. Add a special option to skip this build. This can be
useful when the system dtc is up-to-date, as it speeds up the build.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/builderthread.py b/tools/buildman/builderthread.py
index 5565848..b5afee6 100644
--- a/tools/buildman/builderthread.py
+++ b/tools/buildman/builderthread.py
@@ -406,7 +406,7 @@
                     the next incremental build
         """
         # Set up the environment and command line
-        env = self.toolchain.MakeEnvironment(self.builder.full_path)
+        env = self.builder.make_environment(self.toolchain)
         mkdir(out_dir)
 
         args, cwd, src_dir = self._build_args(brd, out_dir, out_rel_dir,
@@ -574,7 +574,7 @@
                 outf.write(f'{result.return_code}')
 
             # Write out the image and function size information and an objdump
-            env = result.toolchain.MakeEnvironment(self.builder.full_path)
+            env = self.builder.make_environment(self.toolchain)
             with open(os.path.join(build_dir, 'out-env'), 'wb') as outf:
                 for var in sorted(env.keys()):
                     outf.write(b'%s="%s"' % (var, env[var]))