Netduino: Fix the memory region size error

Thanks to Peter C for pointing this out

Signed-off-by: Alistair Francis <alistair23@gmail.com>
Netduino_USART: Prepare for upstreaming

Replaced all memory offsets with #define constants

Also the USART controller now performs some checks on
reading and writing as well as enabled bits (more still
needs to be done, but it's getting better)

Signed-off-by: Alistair Francis <alistair23@gmail.com>
Netduino_GPIO: Replace all memory offsets with constants

Signed-off-by: Alistair Francis <alistair23@gmail.com>
Netduino_SYSCFG: Replace all memory regions with constants

Signed-off-by: Alistair Francis <alistair23@gmail.com>