target_arm: Allow the machine to set the rom reset address

And update Netduino Plus 2 to use this change

Signed-off-by: Alistair Francis <alistair23@gmail.com>
Netduino: Correctly attach devices

Now GPIO, TIM2 to 5, and USARTs are all correctly attached

Signed-off-by: Alistair Francis <alistair23@gmail.com>
Increase the maximum number of serial devices to 6

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