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/ipmi.cpp b/ipmi.cpp
index cdf3b64..2b71315 100644
--- a/ipmi.cpp
+++ b/ipmi.cpp
@@ -21,6 +21,7 @@
 #include "cpld.hpp"
 #include "entity_name.hpp"
 #include "eth.hpp"
+#include "flash_size.hpp"
 #include "handler.hpp"
 #include "machine_name.hpp"
 #include "pcie_i2c.hpp"
@@ -69,6 +70,8 @@
         case SysPsuHardResetOnShutdown:
             return psuHardResetOnShutdown(reqBuf, replyCmdBuf, dataLen,
                                           handler);
+        case SysGetFlashSize:
+            return getFlashSize(reqBuf, replyCmdBuf, dataLen, handler);
         default:
             std::fprintf(stderr, "Invalid subcommand: 0x%x\n", reqBuf[0]);
             return IPMI_CC_INVALID;