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.hpp b/handler.hpp
index 2db83df..1a2cbae 100644
--- a/handler.hpp
+++ b/handler.hpp
@@ -74,6 +74,14 @@
                                       std::uint8_t instance) = 0;
 
     /**
+     * Return the flash size of bmc chip.
+     *
+     * @return the flash size of bmc chip
+     * @throw IpmiException on failure.
+     */
+    virtual uint32_t getFlashSize() = 0;
+
+    /**
      * Return the name of the machine, parsed from release information.
      *
      * @return the machine name