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/commands.hpp b/commands.hpp
index 19632ef..d9efb4f 100644
--- a/commands.hpp
+++ b/commands.hpp
@@ -25,6 +25,8 @@
     SysMachineName = 7,
     // Arm for psu reset on host shutdown
     SysPsuHardResetOnShutdown = 8,
+    // The Sys get flash size command
+    SysGetFlashSize = 9,
 };
 
 } // namespace ipmi