[STM32F4-Discovery (STM32F407VG)] initial port

Hello,

I've ported mbed-sdk to STM32F4-Discovery(STM32F407VG) board.
This port is basically derived from NUCLEO_401RE port
and ported only with GCC_ARM toolchain at the momemt.

The target name is 'DISCO_F407VG'.

The statement at this moment is as follows,
I've checked working well,
(1) Simple 'ticker' program 'MBED_11: Ticker with printf() out to UART' is ok.
PA2: UART TX
PA3: UART RX
(3) MBED_5: PWM
PWM port is PB_3
Period 20msec, duty 50% is ok.
(4) wait_us() function, 100usec is ok.

NUCLEO_401RE(STM32F401RE) and STM32F4-Discovery(STM32F407VG) have
very similar peripheral architecture,
so probably some other peripheral test will be pass.

Build method:
(1) Build mbed library 
python workspace_tools/build.py -m DISCO_F407VG -t GCC_ARM
(2) Generate test project (i.e.)
python workspace_tools/project.py -m DISCO_F407VG -i gcc_arm -p 39
(3) Build test project
Unzip
build/export/MBED_11_gcc_arm_DISCO_F407VG.zip
and execute command line 'make'.

Would you merge this implimentation ?

dinau