USB serial is now working for F7.

Internal file storage is now working for F7.  The flash is laid out a bit
differently to the F4 - 4 x 32K, 1 x 128K with the rest 256K, so the
internal storage is 96K.

Added more pind definitions for STM32F7DISC board.  Made USART1 be the
default HWUART repl.  The STLINK usb connector also looks like a USB
serial port which is attached to USART1 on the STM32F7DISC.