Re-export Interrupt, NVIC_PRIO_BITS. Feature-gate SysTick definition.

This adds two new changes for the `rtic` feature:

1. Re-export the `Interrupt` and `NVIC_PRIO_BITS` as `rtic` requires
   these for generated code related to interrupts.
2. Remove the `SysTick` definition to remove duplicating the `rtic`
   definition.

src/lib.rs

@@ -112,6 +112,10 @@ pub mod usb;
 
 pub use hal::ral::interrupt;
 
+// `rtic` expects these in the root.
+#[cfg(feature = "rtic")]
+pub use hal::ral::{interrupt as Interrupt, NVIC_PRIO_BITS};
+
 pub use cortex_m_rt as rt;
 pub use imxrt_hal as hal;
 
@@ -351,12 +355,11 @@ pub extern "C" fn delay(millis: u32) {
 
 /// Scoping of data related to SYSTICK
 mod systick {
-    use crate::rt::exception;
-
     #[no_mangle]
     static mut systick_millis_count: u32 = 0;
 
-    #[exception]
+    #[cfg(not(feature = "rtic"))]
+    #[crate::rt::exception]
     fn SysTick() {
         unsafe {
             let ms = core::ptr::read_volatile(&systick_millis_count);