update_engine: Add hardware support for firmware_max_rollforward

- Adds the methods to get and set firmware_max_rollforward
- This is for supporting the rollback feature
- A future CL will set these values based on policy

BUG=chromium:840432
TEST=unittests

Change-Id: I560f4cef9595d4a52d011fcfeb7b2675a096aefa
Reviewed-on: https://chromium-review.googlesource.com/1062766
Commit-Ready: Zentaro Kavanagh <zentaro@chromium.org>
Tested-by: Zentaro Kavanagh <zentaro@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/hardware_android.h b/hardware_android.h
index 981f033..b7a6f96 100644
--- a/hardware_android.h
+++ b/hardware_android.h
@@ -44,6 +44,8 @@
   std::string GetECVersion() const override;
   int GetMinKernelKeyVersion() const override;
   int GetMinFirmwareKeyVersion() const override;
+  int GetMaxFirmwareKeyRollforward() const override;
+  bool SetMaxFirmwareKeyRollforward(int firmware_max_rollforward) override;
   bool SetMaxKernelKeyRollforward(int kernel_max_rollforward) override;
   int GetPowerwashCount() const override;
   bool SchedulePowerwash() override;