tools: binman: main.py: add build-sandbox in sys.path

Adds build-sandbox in sys.path to look for libfdt,
otherwise py_test can't use binman.

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/main.py b/tools/binman/main.py
index ab25b48..9392b59 100755
--- a/tools/binman/main.py
+++ b/tools/binman/main.py
@@ -41,6 +41,7 @@
 # Bring in the libfdt module
 sys.path.insert(2, 'scripts/dtc/pylibfdt')
 sys.path.insert(2, os.path.join(srctree, 'scripts/dtc/pylibfdt'))
+sys.path.insert(2, os.path.join(srctree, 'build-sandbox/scripts/dtc/pylibfdt'))
 sys.path.insert(2, os.path.join(srctree, 'build-sandbox_spl/scripts/dtc/pylibfdt'))
 
 # When running under python-coverage on Ubuntu 16.04, the dist-packages