update to #define

Board-STM32F407-Discovery.md

@@ -15,7 +15,8 @@ Cortex M4 STM32F407VG
 ###Status###
 * Initial support has been implemented, user switch and LEDs are supported.
 * Does not have separate directory - folded into initial stm port.
-* To build for STM32F407 Discovery compile with <code>make TARGET=STM32F4DISC</code>.
+* To build for STM32F407 Discovery compile with `make TARGET=STM32F4DISC`.
+* Also set `#define STM32F4DISC` in mpconfigport.h
 
 ###Programming from Linux (via DFU)###
 This section covers how to flash the STM32F4DISCOVERY from linux (I was using Linux Mint 14, which is derived from ubuntu 12.10).