libboard_zynq::stdio: add drop_uart()

drops locks and causes reinitialization which is needed for exceptions
and clock canges.

libboard_zc706/src/abort.rs

@@ -1,13 +1,17 @@
-use libboard_zynq::println;
+use libboard_zynq::{println, stdio};
 
 #[no_mangle]
 pub unsafe extern "C" fn PrefetchAbort() {
+    stdio::drop_uart();
+
     println!("PrefetchAbort");
     loop {}
 }
 
 #[no_mangle]
 pub unsafe extern "C" fn DataAbort() {
+    stdio::drop_uart();
+
     println!("DataAbort");
     loop {}
 }

libboard_zynq/src/stdio.rs

@@ -10,6 +10,10 @@ pub fn get_uart<'a>() -> MutexGuard<'a, LazyUart> {
     unsafe { UART.lock() }
 }
 
+pub fn drop_uart() {
+    unsafe { UART = Mutex::new(LazyUart::Uninitialized); }
+}
+
 /// Initializes the UART on first use through `.deref_mut()` for debug
 /// output through the `print!` and `println!` macros.
 pub enum LazyUart {