Add OEM command to request flash size

The BMC flash is `/dev/mtd0` and the size can found in
`/sys/class/mtd/mtd0/size`. This OEM command will request that
information with MTDINFO.

Tested:
Successfully requested the flash size information with ipmitool.

```
cat /sys/class/mtd/mtd0/size
67108864
```
67108864 / 1024 / 1024 = 64MB flash

```
ipmitool raw 0x2e 0x32 0x79 0x2b 0x00 0x09
 79 2b 00 09 00 00 00 04
```
Output in little endian.
0x04000000 -> 67108864

Change-Id: Iec1b33503d1166a42ceef4b8491e5c19c3a077fe
Signed-off-by: Willy Tu <wltu@google.com>
diff --git a/handler_impl.hpp b/handler_impl.hpp
index f1ea491..677efa2 100644
--- a/handler_impl.hpp
+++ b/handler_impl.hpp
@@ -30,6 +30,7 @@
     void psuResetDelay(std::uint32_t delay) const override;
     void psuResetOnShutdown() const override;
     std::string getEntityName(std::uint8_t id, std::uint8_t instance) override;
+    uint32_t getFlashSize() override;
     std::string getMachineName() override;
     void buildI2cPcieMapping() override;
     size_t getI2cPcieMappingSize() const override;