Add new variable SCAN_EXCLUDE_DIRS; specifies directories to exclude when searching source tree.

These directories are excluded in addition to OUT_DIR.
This can be useful if your build system has other output directories
beyond what OUT_DIR is set to.

Change-Id: I6d98a85bcc8c89279e939406a7fec32547e8922f
diff --git a/core/cleanspec.mk b/core/cleanspec.mk
index 500ce54..4441e2a 100644
--- a/core/cleanspec.mk
+++ b/core/cleanspec.mk
@@ -64,6 +64,6 @@
 # ************************************************
 
 subdir_cleanspecs := \
-    $(shell build/tools/findleaves.py --prune=$(OUT_DIR) --prune=.repo --prune=.git . CleanSpec.mk)
+    $(shell build/tools/findleaves.py $(FIND_LEAVES_EXCLUDES) . CleanSpec.mk)
 include $(subdir_cleanspecs)
 subdir_cleanspecs :=