syscfg: Update to properly act on interrupts

Signed-off-by: Alistair Francis <alistair23@gmail.com>
external_access: Correctly zero the values

Signed-off-by: Alistair Francis <alistair23@gmail.com>
syscfg: Update EXTI checking

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