"Steal" single value variables in rbc inheritance

This behavior of "stealing" values from parent makefiles is needed to
match make. We already had similar behavior for list variables via
the __move_items function, but were missing it for single value
variables.

Test: ./out/rbcrun -mode rbc ./build/make/tests/run.rbc
Change-Id: Ib320b9b1cce0224184f585c7a391be1b5353b440
diff --git a/tests/run.rbc b/tests/run.rbc
index 85d6c09..221b40f 100644
--- a/tests/run.rbc
+++ b/tests/run.rbc
@@ -26,6 +26,7 @@
 load(":board.rbc", board_init = "init")
 load(":board_input_vars.rbc", board_input_vars_init = "init")
 load("//build/make/tests/single_value_inheritance:test.rbc", test_single_value_inheritance = "test")
+load("//build/make/tests/single_value_inheritance_2:test.rbc", test_single_value_inheritance_2 = "test")
 load("//build/make/tests/artifact_path_requirements:test.rbc", test_artifact_path_requirements = "test")
 load("//build/make/tests/prefixed_sort_order:test.rbc", test_prefixed_sort_order = "test")
 load("//build/make/tests/inherits_in_regular_variables:test.rbc", test_inherits_in_regular_variables = "test")
@@ -181,6 +182,7 @@
 assert_eq("", g.get("NEWVAR"))
 
 test_single_value_inheritance()
+test_single_value_inheritance_2()
 test_artifact_path_requirements()
 test_prefixed_sort_order()
 test_inherits_in_regular_variables()