Implement abspath

This is to minimize the differences between the make
and starlark product configs for the quick ci test.

Bug: 229132189
Test: Manually
Change-Id: I3286ba64e9d8fc9d1b169a7ed18f5a565a117a8d
diff --git a/core/product_config.rbc b/core/product_config.rbc
index 917475a..928cc55 100644
--- a/core/product_config.rbc
+++ b/core/product_config.rbc
@@ -403,7 +403,9 @@
 
 def _abspath(path):
     """Provided for compatibility, to be removed later."""
-    return path
+    if type(path) == "list":
+        path = " ".join(path)
+    return rblf_shell("realpath "+path)
 
 
 def _addprefix(prefix, string_or_list):