Added a new demo to initialise SDRAM, use USART rather than USB serial

Note that there is a nasty hack:
```
//FIXME this should not be needed
#define STM32F429_439xx
```
In the ST .h files (FMC and discovery sdram files), otherwise I get an error in stm32f4xx.h 

However with the hack this runs fine, SDRAM seems quite fast.