update_engine: Add GetMinFirmwareKeyVersion to read tpm_fwver value

Add function to read the minimum allowed firmware key version, ie.
crossystem tpm_fwver.

This is the firmware key version equivalent of crrev.com/c/927827.

BUG=chromium:840432
TEST='cros_run_unit_tests --board=caroline --packages update_engine'

Change-Id: Iadd56b3283028722c94e4ebc7cf5b6470cf1fec9
Reviewed-on: https://chromium-review.googlesource.com/1049667
Commit-Ready: Marton Hunyady <hunyadym@chromium.org>
Tested-by: Marton Hunyady <hunyadym@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
diff --git a/common/mock_hardware.h b/common/mock_hardware.h
index 03ebfdb..fcbb0d5 100644
--- a/common/mock_hardware.h
+++ b/common/mock_hardware.h
@@ -57,6 +57,9 @@
     ON_CALL(*this, GetMinKernelKeyVersion())
         .WillByDefault(
             testing::Invoke(&fake_, &FakeHardware::GetMinKernelKeyVersion));
+    ON_CALL(*this, GetMinFirmwareKeyVersion())
+        .WillByDefault(
+            testing::Invoke(&fake_, &FakeHardware::GetMinFirmwareKeyVersion));
     ON_CALL(*this, SetMaxKernelKeyRollforward())
         .WillByDefault(
             testing::Invoke(&fake_, &FakeHardware::SetMaxKernelKeyRollforward));
@@ -88,6 +91,7 @@
   MOCK_CONST_METHOD0(GetFirmwareVersion, std::string());
   MOCK_CONST_METHOD0(GetECVersion, std::string());
   MOCK_CONST_METHOD0(GetMinKernelKeyVersion, int());
+  MOCK_CONST_METHOD0(GetMinFirmwareKeyVersion, int());
   MOCK_CONST_METHOD1(SetMaxKernelKeyRollforward,
                      bool(int max_kernel_rollforward));
   MOCK_CONST_METHOD0(GetPowerwashCount, int());