buildman: Use out-env for environment output

At present the environment used by U-Boot is written to the 'env'
directory. This is fine when the output directory is not the same as the
source directory, but when it is (as with -w) it conflicts with the source
directory of the same name.

Rename 'env' to 'out-env' to fix this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/builderthread.py b/tools/buildman/builderthread.py
index fc6e1ab..063bbc0 100644
--- a/tools/buildman/builderthread.py
+++ b/tools/buildman/builderthread.py
@@ -333,7 +333,7 @@
 
             # Write out the image and function size information and an objdump
             env = result.toolchain.MakeEnvironment(self.builder.full_path)
-            with open(os.path.join(build_dir, 'env'), 'w') as fd:
+            with open(os.path.join(build_dir, 'out-env'), 'w') as fd:
                 for var in sorted(env.keys()):
                     print('%s="%s"' % (var, env[var]), file=fd)
             lines = []