read_tree_values: more robust parsing.

Skip entries not containing a ":".
diff --git a/devlib/target.py b/devlib/target.py
index ec24765..6d2a12d 100644
--- a/devlib/target.py
+++ b/devlib/target.py
@@ -620,7 +620,7 @@
                                     check_exit_code=check_exit_code)
         result = {}
         for entry in output.strip().split('\n'):
-            if not entry.strip():
+            if ':' not in entry:
                 continue
             path, value = entry.strip().split(':', 1)
             result[path] = value