Retarget STDIO to other than UART_2 without the need to patch the mbed source code

I have added ifndef directives to be able to override the default STDIO_UART* macro values at build time. This enables overrides without the need to patch the mbed source code (after this pull request is accepted).

I need such an override to retarget STDIO to other than the default UART_2 device, when using STM32F4-Discovery with STM32F4-BaseBoard. The latter has the RS-232 interface routed to the UART_6 device.

I provide a [project example](https://github.com/istarc/stm32/tree/master/examples/BaseBoardUART.mbed-RetargetSTDIO) with an appropriate [Makefile](https://github.com/istarc/stm32/blob/master/examples/BaseBoardUART.mbed-RetargetSTDIO/Makefile) \(see DEFS variable\) to demonstrate this pull request, 

This pull request updates only the STM32F407VG code. Other targets may need this update as well.