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/Makefile.am b/Makefile.am
index 34e60bd..5016449 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -44,6 +44,7 @@
 	cable.cpp \
 	cpld.cpp \
 	eth.cpp \
+	flash_size.cpp \
 	psu.cpp \
 	pcie_i2c.cpp \
 	entity_name.cpp \