Move variable assignment handling to generation context

This allows the parsing code to be cleaner, as it
doesn't have to care about variable assignments.

Bug: 228518745
Test: go test
Change-Id: I33425c2fb51acab4901bfa82a53d337b75210f8e
diff --git a/mk2rbc/mk2rbc_test.go b/mk2rbc/mk2rbc_test.go
index 3698813..33961f1 100644
--- a/mk2rbc/mk2rbc_test.go
+++ b/mk2rbc/mk2rbc_test.go
@@ -626,7 +626,7 @@
     pass
   elif not rblf.board_platform_is(g, "copper"):
     pass
-  elif g.get("TARGET_BOARD_PLATFORM", "") not in g["QCOM_BOARD_PLATFORMS"]:
+  elif g.get("TARGET_BOARD_PLATFORM", "") not in g.get("QCOM_BOARD_PLATFORMS", ""):
     pass
   elif g["TARGET_PRODUCT"] in g.get("PLATFORM_LIST", []):
     pass
@@ -649,7 +649,7 @@
     pass
   elif not rblf.board_platform_is(g, "copper"):
     pass
-  elif g.get("TARGET_BOARD_PLATFORM", "") in g["QCOM_BOARD_PLATFORMS"]:
+  elif g.get("TARGET_BOARD_PLATFORM", "") in g.get("QCOM_BOARD_PLATFORMS", ""):
     pass
 `,
 	},